Hi,
Recently got hold of a half working(1 channel is working) tube amp for cheap hoping its something i can fix myself. But unlucky me, it end up being a custom made IC that does the auto biasing for the output tube that is blown. and a replacement piece is not available. So I'm thinking of re-using the chassis and the pair of output transformer.
Did some quick test with a variac and got the following data:
30:1 turn ratio
So..
5400 ohm into 6ohm(amp is originally advertise as 6ohm output)?
However, the construction of the output transformer look bit strange to me (maybe only to me) so i was hoping i can make some changes to how it is wired so i can use it for other tube instead of sticking to push pull el34 designs. Reason being the output transformer doesn't seem to have any ultra linear tabs and most el34 designs are UL and triode mode doesn't seem to give me enough power.
Primary:
The first picture below shows the primary 2 x purple to plate yellow tied together for B+. It looks like a dual bobbin? I don't seem to be able to figure out why is it wired this way. Or is blue actually the UL tab?
Fiddling with a DMM, shows that for the primary, Each pair (blue & blue, Yellow & Yellow, Purple & Purple) is actually isolated so i actually have 6 set of winding 3 set on each bobbin?
Secondary:
The second picture shows the top of the secondary and the bottom is a mirror image. The third picture is how it is wired as 6ohm. Is there any chance i can change the configuration so i can somehow get a 16ohm secondary to drive my LS3/5A?
Fiddling with a DMM, shows that for the secondary, Each pair is actually isolated just like the primary.
From my questions, its very obvious I'm a newbie however I'm willing to learn more about my transformer so i can use it properly. From some research the transformer suppose to be quite hard to wind because of the multiple windings so i assume its pretty good quality?
I can do some simple test with DMM and Variac so i would appreciate if anyone could guide me how i can further explore the wiring configuration for this pair of output transformer?
Thank you in advance!
Below is the link to the amplifier (VA 1.0):
Cadence VA 1.0 Amplifier
-Askae
Recently got hold of a half working(1 channel is working) tube amp for cheap hoping its something i can fix myself. But unlucky me, it end up being a custom made IC that does the auto biasing for the output tube that is blown. and a replacement piece is not available. So I'm thinking of re-using the chassis and the pair of output transformer.
Did some quick test with a variac and got the following data:
30:1 turn ratio
So..
5400 ohm into 6ohm(amp is originally advertise as 6ohm output)?
However, the construction of the output transformer look bit strange to me (maybe only to me) so i was hoping i can make some changes to how it is wired so i can use it for other tube instead of sticking to push pull el34 designs. Reason being the output transformer doesn't seem to have any ultra linear tabs and most el34 designs are UL and triode mode doesn't seem to give me enough power.
Primary:
The first picture below shows the primary 2 x purple to plate yellow tied together for B+. It looks like a dual bobbin? I don't seem to be able to figure out why is it wired this way. Or is blue actually the UL tab?
Fiddling with a DMM, shows that for the primary, Each pair (blue & blue, Yellow & Yellow, Purple & Purple) is actually isolated so i actually have 6 set of winding 3 set on each bobbin?
Secondary:
The second picture shows the top of the secondary and the bottom is a mirror image. The third picture is how it is wired as 6ohm. Is there any chance i can change the configuration so i can somehow get a 16ohm secondary to drive my LS3/5A?
Fiddling with a DMM, shows that for the secondary, Each pair is actually isolated just like the primary.
From my questions, its very obvious I'm a newbie however I'm willing to learn more about my transformer so i can use it properly. From some research the transformer suppose to be quite hard to wind because of the multiple windings so i assume its pretty good quality?
I can do some simple test with DMM and Variac so i would appreciate if anyone could guide me how i can further explore the wiring configuration for this pair of output transformer?
Thank you in advance!
Below is the link to the amplifier (VA 1.0):
Cadence VA 1.0 Amplifier
-Askae
Attachments
Unless you want the "iron" wrecked too, leave it alone.
The way to deal with your power issue is by employing full pentode mode "finals" and regulating g2 B+ at a large fraction of plate B+.IMO, the best available EL34 class tube is the "reissue" Gold Lion KT77. If your budget can take the "hit", use 'em.
If Eli says to leave the transformers alone, I would follow that advice!
I looked at the Cadence amp info, but didn't learn very much.
Is it a hybrid amp with SS input stages and tube outputs?
Designed to drive electrostatic speakers?
Drawing the whole amp schematic - if it can't be found online, or even drawing the output stage schematic for one channel (and posting it here) might get you some advice on how to get the amp going again without a complete rebuild to a different design.
I looked at the Cadence amp info, but didn't learn very much.
Is it a hybrid amp with SS input stages and tube outputs?
Designed to drive electrostatic speakers?
Drawing the whole amp schematic - if it can't be found online, or even drawing the output stage schematic for one channel (and posting it here) might get you some advice on how to get the amp going again without a complete rebuild to a different design.
Just because you lost a "custom" IC doesn't mean you can't fix it, and find some other way to bias the finals to get it working again.
If you don't want to do that, and are determined to recycle the OPTs, then there is another possibility other than EL34s. Although with the other channel OK, I'd consider a restoration before parting it out.
5K4 (P-2-P) is a very close match for the 10JA5. That'll get you just under 35W, and you just might be able to avoid UL. These TV deflection types tend to not make the nastier higher order harmonics, and make mostly h3. You can add just enough gNFB to take the "edge" off and tun them as full on pents: no trioding needed (and not possible, given the screen voltage limitations) or UL or other local NFB.
If you don't want to do that, and are determined to recycle the OPTs, then there is another possibility other than EL34s. Although with the other channel OK, I'd consider a restoration before parting it out.
5K4 (P-2-P) is a very close match for the 10JA5. That'll get you just under 35W, and you just might be able to avoid UL. These TV deflection types tend to not make the nastier higher order harmonics, and make mostly h3. You can add just enough gNFB to take the "edge" off and tun them as full on pents: no trioding needed (and not possible, given the screen voltage limitations) or UL or other local NFB.
Attachments
I took a look at the linked info. The O/P trafos are definitely reusable, with full pentode mode "finals". The power trafo is a question mark. What other windings, other the tube B+ and filament, are present? Can the additional windings be put to good use? What are the black "dodads" on top of the chassis? The photo in the link is not clear. Some good photos of the top (with the back cover removed), the back panel, and the underside will help us "scheme and plot".
It looks like enough top plate room is present for 2X Noval (9 pin mini) sockets (1/channel) to be added. Dyna/Fisher/Scott style circuitry with a 6922 in cascode as the voltage amplifier and an IRFBC20 MOSFET as the DC coupled "concertina" phase splitter should fit mechanically well.
I've uploaded a "hen scratch" schematic of a 6922 cascode which might be a "place of departure" for the new small signal circuitry.
It looks like enough top plate room is present for 2X Noval (9 pin mini) sockets (1/channel) to be added. Dyna/Fisher/Scott style circuitry with a 6922 in cascode as the voltage amplifier and an IRFBC20 MOSFET as the DC coupled "concertina" phase splitter should fit mechanically well.
I've uploaded a "hen scratch" schematic of a 6922 cascode which might be a "place of departure" for the new small signal circuitry.
Attachments
Thank you so much for the response and advices so far its amazing gotta love DIYAUDIO community.
Some information first.
Attached is some nudies of the amp. Yes it is a hybrid tube amp with auto biasing through SS as well as the power supply and also yes it is meant to drive electrostatic speakers.
Power tranny is a toroidal with 0-320, 0-320, 0-6.3, 0-6.3, 0-15, 0-15
Intend to mate this "new" power amp with a CJ PV-10A and Rogers ls3/5a
Some ideas i'm toying with:
1) Pete millett uniamp 2: quite costly to build can only use the triode mode and the circuit board may not fit the chassis.
2) Pointdexter el34 amp: triode mode power
3) Allen wright pp1c: triode mode power
What i think:
Actually this amp is more of an experimental amp and its my first tube project after building some solid state stuffs (dcb1 honey badger etc..) wanted to make it a full tube amp but don't mind SS power supply. The only thing that attracted me in this tube amp is the output transformer, chassis(heavy!), winged "C" el34 tube. The rest don't seem very well done up to my inexperience eyes.
I don't mind making some modification to chassis etc as I'm going to convert it into a power amp anyway so out goes the pots and selectors.
Actually its fine for me to use the amp as triode connected as it is a bedroom setup and i don't listen very loudly but i have quite some restriction because of the output tranny and speaker load.
6 ohm secondary will be an issue with my 15ohm ls3/5a. using a 15ohm speaker to 6 ohm output i will have almost over 15k impedance? which means lots of power will be needed not even talking about ls3/5a being not very efficient.
So I'm now actually at the crossroad so in order to help me in making some decisions, i would like to know if i can do any work to my output transformer wiring first which will make my decision making task easier.
- 16 ohm output possible?
- UL tap possible?
If no changes could be made to the output tranny, there is only 4 ways to go.
1) Convert it to pentode output as eli duttman suggested but there isn't alot of pentode el34 designs forumers rave about.
2) Changing to KT88 to get more power with a new power tranny
3) build either of the triode output design and hope its good enough to drive the ls3/5a if not keep it as a spare amp.
4) Scrap the tubes and work on a real triode. 300b p-p work with this output tranny? most of them suggested around 3k impedance.
Sorry for the long post but it will provide enough information for good suggestion. Thank you in advance!
Askae
Some information first.
Attached is some nudies of the amp. Yes it is a hybrid tube amp with auto biasing through SS as well as the power supply and also yes it is meant to drive electrostatic speakers.
Power tranny is a toroidal with 0-320, 0-320, 0-6.3, 0-6.3, 0-15, 0-15
Intend to mate this "new" power amp with a CJ PV-10A and Rogers ls3/5a
Some ideas i'm toying with:
1) Pete millett uniamp 2: quite costly to build can only use the triode mode and the circuit board may not fit the chassis.
2) Pointdexter el34 amp: triode mode power
3) Allen wright pp1c: triode mode power
What i think:
Actually this amp is more of an experimental amp and its my first tube project after building some solid state stuffs (dcb1 honey badger etc..) wanted to make it a full tube amp but don't mind SS power supply. The only thing that attracted me in this tube amp is the output transformer, chassis(heavy!), winged "C" el34 tube. The rest don't seem very well done up to my inexperience eyes.
I don't mind making some modification to chassis etc as I'm going to convert it into a power amp anyway so out goes the pots and selectors.
Actually its fine for me to use the amp as triode connected as it is a bedroom setup and i don't listen very loudly but i have quite some restriction because of the output tranny and speaker load.
6 ohm secondary will be an issue with my 15ohm ls3/5a. using a 15ohm speaker to 6 ohm output i will have almost over 15k impedance? which means lots of power will be needed not even talking about ls3/5a being not very efficient.
So I'm now actually at the crossroad so in order to help me in making some decisions, i would like to know if i can do any work to my output transformer wiring first which will make my decision making task easier.
- 16 ohm output possible?
- UL tap possible?
If no changes could be made to the output tranny, there is only 4 ways to go.
1) Convert it to pentode output as eli duttman suggested but there isn't alot of pentode el34 designs forumers rave about.
2) Changing to KT88 to get more power with a new power tranny
3) build either of the triode output design and hope its good enough to drive the ls3/5a if not keep it as a spare amp.
4) Scrap the tubes and work on a real triode. 300b p-p work with this output tranny? most of them suggested around 3k impedance.
Sorry for the long post but it will provide enough information for good suggestion. Thank you in advance!
Askae
Attachments
UL is out. You'll destroy the O/P "iron", if you try to modify it.
You might be able to use the LS3/5A speakers by buying, or winding yourself, toroid impedance matchers. I don't know about current availability, but such things have definitely been available in the market.
Stop worrying about full pentode mode "finals".
Regulate O/P tube g2 B+ at a large fraction of O/P tube anode B+ and carefully tune the NFB. You'll do just fine. Study the EL34 data sheet and pay close attention to "fixed" bias operating condition sets. The 320 VAC available from the OEM power trafo will limit the B+ rail to something under the 452.5 V. IMO, that's not enough to go messing around with cathode (self) bias. The rail you get should be "tall" enough to get the regulated B+ a cascode voltage amplifier, with its poor PSR, requires. I suggest a cascode voltage amplifier and a MOSFET phase splitter as all of the popular/useful pentode/triodes are out of production. Also, the high transconductance of the recommended devices will give you some resistance to slew limiting induced by the NFB's HF error correction signal. Another good reason for a little "sand" is reducing the size of the additional filament trafo that will be needed.
Phase those 15 VAC windings up, connect them in series, and bridge rectify, with Schottky diodes, to get a bias supply.
=C= EL34s are good tubes. It's a shame they are no longer being made. Use them until they wear out.
You might be able to use the LS3/5A speakers by buying, or winding yourself, toroid impedance matchers. I don't know about current availability, but such things have definitely been available in the market.
Stop worrying about full pentode mode "finals".
Regulate O/P tube g2 B+ at a large fraction of O/P tube anode B+ and carefully tune the NFB. You'll do just fine. Study the EL34 data sheet and pay close attention to "fixed" bias operating condition sets. The 320 VAC available from the OEM power trafo will limit the B+ rail to something under the 452.5 V. IMO, that's not enough to go messing around with cathode (self) bias. The rail you get should be "tall" enough to get the regulated B+ a cascode voltage amplifier, with its poor PSR, requires. I suggest a cascode voltage amplifier and a MOSFET phase splitter as all of the popular/useful pentode/triodes are out of production. Also, the high transconductance of the recommended devices will give you some resistance to slew limiting induced by the NFB's HF error correction signal. Another good reason for a little "sand" is reducing the size of the additional filament trafo that will be needed. Phase those 15 VAC windings up, connect them in series, and bridge rectify, with Schottky diodes, to get a bias supply.
=C= EL34s are good tubes. It's a shame they are no longer being made.
Use them until they wear out.ELI gave you sound advice and it will be much better than UL and you won't destroy your amp.. Regulated screen grids running pure pentode is the way to go..AAMOF, the manufacturers recommend this in their data.Hi,
Recently got hold of a half working(1 channel is working) tube amp for cheap hoping its something i can fix myself. But unlucky me, it end up being a custom made IC that does the auto biasing for the output tube that is blown. and a replacement piece is not available. So I'm thinking of re-using the chassis and the pair of output transformer.
Did some quick test with a variac and got the following data:
30:1 turn ratio
So..
5400 ohm into 6ohm(amp is originally advertise as 6ohm output)?
However, the construction of the output transformer look bit strange to me (maybe only to me) so i was hoping i can make some changes to how it is wired so i can use it for other tube instead of sticking to push pull el34 designs. Reason being the output transformer doesn't seem to have any ultra linear tabs and most el34 designs are UL and triode mode doesn't seem to give me enough power.
Primary:
The first picture below shows the primary 2 x purple to plate yellow tied together for B+. It looks like a dual bobbin? I don't seem to be able to figure out why is it wired this way. Or is blue actually the UL tab?
Fiddling with a DMM, shows that for the primary, Each pair (blue & blue, Yellow & Yellow, Purple & Purple) is actually isolated so i actually have 6 set of winding 3 set on each bobbin?
Secondary:
The second picture shows the top of the secondary and the bottom is a mirror image. The third picture is how it is wired as 6ohm. Is there any chance i can change the configuration so i can somehow get a 16ohm secondary to drive my LS3/5A?
Fiddling with a DMM, shows that for the secondary, Each pair is actually isolated just like the primary.
From my questions, its very obvious I'm a newbie however I'm willing to learn more about my transformer so i can use it properly. From some research the transformer suppose to be quite hard to wind because of the multiple windings so i assume its pretty good quality?
I can do some simple test with DMM and Variac so i would appreciate if anyone could guide me how i can further explore the wiring configuration for this pair of output transformer?
Thank you in advance!
Below is the link to the amplifier (VA 1.0):
Cadence VA 1.0 Amplifier
-Askae
With A 5400 OHM primary,gives you some expanded options but I would guess its normally 5k impedance, The KT77 is a great tube but you also run KT66s with that impedance but you need a little more drive...
I wouldn't try tearing up the transformers but running them in pure pentode you will gain a wonderful sonic advantage provided you regulate G2 and drop its voltage.
Thank you for all the responses so far. Really appreciate it.
Actually I'm not too stubborn is changing the output transformer into get a UL tap. I'm more curious if I could do anything to the secondary to be wired to into a 16ohm or something close so I can hook it up to my ls3/5a and not having too much impedance in the primary? If I can match out the secondary I can live with triode output as my room is pretty small and I don't need that much power.
Some research on output transformer shows that some of the traffo output with multiple winding are actually each at 1ohm. So wiring them up in series may result to a desired resistance. Is there anyway I could check if the secondary winding of my traffo is actually constructed in this way or we have to stick to the factory wired. I only have a variac and a dmm to do any test. The last test I did is to confirm that the secondary is actually 6 pair of separate windings 3 on each bobbin.
Lastly, sorry to ask for some spoon feeding. I tried googling for a good pentode final el34 schematic but couldn't find one even with a full tube setup a hybrid will be even tougher. I'm not good enough to read some topology and create a schematic. I may need a schematic which I can follow as I don't have much experience with tube amps.
Thank you!
Askae
Actually I'm not too stubborn is changing the output transformer into get a UL tap. I'm more curious if I could do anything to the secondary to be wired to into a 16ohm or something close so I can hook it up to my ls3/5a and not having too much impedance in the primary? If I can match out the secondary I can live with triode output as my room is pretty small and I don't need that much power.
Some research on output transformer shows that some of the traffo output with multiple winding are actually each at 1ohm. So wiring them up in series may result to a desired resistance. Is there anyway I could check if the secondary winding of my traffo is actually constructed in this way or we have to stick to the factory wired. I only have a variac and a dmm to do any test. The last test I did is to confirm that the secondary is actually 6 pair of separate windings 3 on each bobbin.
Lastly, sorry to ask for some spoon feeding. I tried googling for a good pentode final el34 schematic but couldn't find one even with a full tube setup a hybrid will be even tougher. I'm not good enough to read some topology and create a schematic. I may need a schematic which I can follow as I don't have much experience with tube amps.
Thank you!
Askae
AFAIK, you are going to have to approach this problem in Chinese American restaurant menu fashion and select 1 from column A and 1 from column B.
The power O/P section of the Fisher 500C receiver is illustrative of voltage amplifier DC coupled to "concertina" phase splitter and pentode mode "finals". The hand drawn schematic I previously uploaded is the voltage amplifier to use, instead of Fisher's triode. An IRFBC20 MOSFET will replace Fisher's triode as the "concertina" phase splitter.
Multiple instances of Maida style circuitry will provide the regulation needed for cascode B+ and O/P tube g2 B+.
The power O/P section of the Fisher 500C receiver is illustrative of voltage amplifier DC coupled to "concertina" phase splitter and pentode mode "finals". The hand drawn schematic I previously uploaded is the voltage amplifier to use, instead of Fisher's triode. An IRFBC20 MOSFET will replace Fisher's triode as the "concertina" phase splitter.
Multiple instances of Maida style circuitry will provide the regulation needed for cascode B+ and O/P tube g2 B+.
Looks like all the secondaries are connected in parallel. If so, 6 Ohms is your only choice. If all the primary sections are equal and connected in series you have 33% (as used by Dyna) and 67% UL ratios available. This could be confirmed by measuring the voltage from one end to each of the taps with a (safe) low voltage applied. DON'T disconnect the primary sections unless you carefully record what connects to what - the transformer response and breakdown voltage depend on how they are connected.
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