Yes, ground the transformer secondary.
Also check your PSU cap connections for a cold solder joint or something similar.
Also check your PSU cap connections for a cold solder joint or something similar.
Hup! Thanks, it was a stupid miss from my drawing and my assembly. Now it tests great,
Thanks again.
Thanks again.
I am coming into this thread really late.
I like that 6L6 amp. good job.
But now, back to the original ST70 comments:
For anybody who has an original ST70:
1. Those black coupling caps (about 1/2 inch by 1.5 inch) All become Very leaky with age.
i.e., one of them caused the EL34 that it drove to have 60mA, while the other EL34 had only 40 mA, with the bias adjusted for 100mA total across the 1.56 Ohm sense resistor.
Original/Pretty is not going to be working good. Replace those coupling caps, even if you change nothing else.
2. The 7199 triode section has too high of a voltage on the cathode. Elevate the center tap of the two 6.3V windings using a resistive divider from B+ to ground, and bypass that with a cap. About 50VDC there will save the input tubes (otherwise the filament to cathode becomes leaky, and returns through the original center tap capacitor to ground, and the 7199 phase splitter cathode and plate signals become unbalanced.
Refer to "Sound Practices" Issue # 10.
I like that 6L6 amp. good job.
But now, back to the original ST70 comments:
For anybody who has an original ST70:
1. Those black coupling caps (about 1/2 inch by 1.5 inch) All become Very leaky with age.
i.e., one of them caused the EL34 that it drove to have 60mA, while the other EL34 had only 40 mA, with the bias adjusted for 100mA total across the 1.56 Ohm sense resistor.
Original/Pretty is not going to be working good. Replace those coupling caps, even if you change nothing else.
2. The 7199 triode section has too high of a voltage on the cathode. Elevate the center tap of the two 6.3V windings using a resistive divider from B+ to ground, and bypass that with a cap. About 50VDC there will save the input tubes (otherwise the filament to cathode becomes leaky, and returns through the original center tap capacitor to ground, and the 7199 phase splitter cathode and plate signals become unbalanced.
Refer to "Sound Practices" Issue # 10.
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RE: Stereohifi
Is the UL tap N Fdbk connected up like the ST70 original? I don't see it shown in your schematic.
Tried any comparison with the UL Fdbk connected versus not connected?
Is the UL tap N Fdbk connected up like the ST70 original? I don't see it shown in your schematic.
Tried any comparison with the UL Fdbk connected versus not connected?
Hi, unfortunately the output transformers on the Fisher 800C did not have the ultra linear points at their time.(1962)
What i'm making is totally from new Digikey parts and driver boards PC3A and new tubes except the for original Fisher excellent transformers.
I do use the feedback from the 16ohms tap and from the screens to the PC3A as Dynaco design it, up to now it is stable.
So I'm using a CRC power supply and drop B+ from 440Vdc to 388Vdc for the screens. They have a 1Kohms screen resistor to each tube.
That also works great my other 6L6 amp with a Triode labs driver boards with Heathkit transformers and again theses don't have UL points on output transformers. This Heathkit is tube rectified, as the Fisher amp is solid state with fast MUR210 diodes.
Will final tune THD with my Sound Technology 1700B for fine tuning square waves and % of THD.
Got a measured 0,15% THD with my Heathkit 6L6 project with Triode Electronics board that have 12AU7 driver tubes.
Thanks
What i'm making is totally from new Digikey parts and driver boards PC3A and new tubes except the for original Fisher excellent transformers.
I do use the feedback from the 16ohms tap and from the screens to the PC3A as Dynaco design it, up to now it is stable.
So I'm using a CRC power supply and drop B+ from 440Vdc to 388Vdc for the screens. They have a 1Kohms screen resistor to each tube.
That also works great my other 6L6 amp with a Triode labs driver boards with Heathkit transformers and again theses don't have UL points on output transformers. This Heathkit is tube rectified, as the Fisher amp is solid state with fast MUR210 diodes.
Will final tune THD with my Sound Technology 1700B for fine tuning square waves and % of THD.
Got a measured 0,15% THD with my Heathkit 6L6 project with Triode Electronics board that have 12AU7 driver tubes.
Thanks
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I wish the Bob Richards link still worked: 404 Not Found
When I tried it I got a Wall-Mart ad for swimming tubes. Via helpful ATT.
I'm diving into my 1961 built ST-70 this spring. The left channel is dead with new JJ output tubes. The B+ is 370, idle current is a little low on the dead channel. Paper coupler caps were changed by me to "arco" polyester in the mid-seventies, as was the selenium rectifier changed to 1n4004. The 7199 are original, always worked up to now, so I've bought both the classicvalve.ca "marantz" driver board (triple 6SN7) and "dynamutt" with dual 6EJ7 6CG7 's . The choke melted all the wax out before I bought it in 1970 but still works okay. I used it 2000 hours a year from 1970 to 1980, then couldn't get new output tubes and shelved it.
As far as sound, after I got the dynaco ST120 working, I found the ST-70 top octave piano and bells to be distorted. And the ST120 is "the worst amp sold on earth" people say. So why I'm jumping to a new driver board. I found the bass quite impressive for 1970, especially on my LWEIII speakers with the feedback kit installed that would make the 10" drivers drive out an inch on certain ASCO 45's.
The new 6SN7 were from JJ, T&M supplied USA labeled GE 6EJ7 & Sylvania 6CG7.
My power transformer never got hot enough to worry about in my non-airconditioning house in Houston, so replacing the 5AR4 with SS would be a technical argument, not the reliability argument listed above.
One thing I liked about the original configuration, it took a lightning strike from the power line in 1981 hard enough to vaporize the turn off pop capacitor and carbon track the power switch to "on" while it was turned off. No other symptoms. Try that with any modern entertainment product. This is another lightning hot spot, so during certain stormy spring days the ST-70 may again be my primary entertainment source.
When I tried it I got a Wall-Mart ad for swimming tubes. Via helpful ATT.
I'm diving into my 1961 built ST-70 this spring. The left channel is dead with new JJ output tubes. The B+ is 370, idle current is a little low on the dead channel. Paper coupler caps were changed by me to "arco" polyester in the mid-seventies, as was the selenium rectifier changed to 1n4004. The 7199 are original, always worked up to now, so I've bought both the classicvalve.ca "marantz" driver board (triple 6SN7) and "dynamutt" with dual 6EJ7 6CG7 's . The choke melted all the wax out before I bought it in 1970 but still works okay. I used it 2000 hours a year from 1970 to 1980, then couldn't get new output tubes and shelved it.
As far as sound, after I got the dynaco ST120 working, I found the ST-70 top octave piano and bells to be distorted. And the ST120 is "the worst amp sold on earth" people say. So why I'm jumping to a new driver board. I found the bass quite impressive for 1970, especially on my LWEIII speakers with the feedback kit installed that would make the 10" drivers drive out an inch on certain ASCO 45's.
The new 6SN7 were from JJ, T&M supplied USA labeled GE 6EJ7 & Sylvania 6CG7.
My power transformer never got hot enough to worry about in my non-airconditioning house in Houston, so replacing the 5AR4 with SS would be a technical argument, not the reliability argument listed above.
One thing I liked about the original configuration, it took a lightning strike from the power line in 1981 hard enough to vaporize the turn off pop capacitor and carbon track the power switch to "on" while it was turned off. No other symptoms. Try that with any modern entertainment product. This is another lightning hot spot, so during certain stormy spring days the ST-70 may again be my primary entertainment source.
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An internal MOV across the AC line (or an external line conditioner) will help also.
No UL taps on the 800C and 500C output transformers, what a pity.
Instead, see how they succeeded:
The Fisher 800C and Fisher 500C had 7591A output tubes.
They were operated in pentode mode, not Ultra Linear.
They only had a wimpy gain stage & split load phase splitter: the 12AX7.
It used global negative feedback from the output transformer back to the cathode circuit of the gain stage.
Because of the limiting factors, it had to have output transformers that were of relatively high quality.
The transformers made it possible to all work together.
Yes, the 800C and 500C amplifier section did quite well for the time, price, and simplicity.
Another synergistic success.
The 500C and 800C output transformers (bare bones, out of circuit) had quite good square wave response and quite good frequency response. So those who could get there hands on them, used them in the Oregon Triode Society Amplifier (DIY), which had two 6C45pi in a cathode coupled splitter with current source in the cathode circuit, Interstage transformer, and individually self biased push pull 300B outputs.
It had No negative feedback, unless you count using a cathode coupled splitter and push pull interstage as having feedback. It legally does have Local negative feedback (but not thought of that way by most casual observers). The cathode to cathode, and plate to plate couplings both linearize the 6C45pi gain/splitter stage's even harmonics.
Instead, see how they succeeded:
The Fisher 800C and Fisher 500C had 7591A output tubes.
They were operated in pentode mode, not Ultra Linear.
They only had a wimpy gain stage & split load phase splitter: the 12AX7.
It used global negative feedback from the output transformer back to the cathode circuit of the gain stage.
Because of the limiting factors, it had to have output transformers that were of relatively high quality.
The transformers made it possible to all work together.
Yes, the 800C and 500C amplifier section did quite well for the time, price, and simplicity.
Another synergistic success.
The 500C and 800C output transformers (bare bones, out of circuit) had quite good square wave response and quite good frequency response. So those who could get there hands on them, used them in the Oregon Triode Society Amplifier (DIY), which had two 6C45pi in a cathode coupled splitter with current source in the cathode circuit, Interstage transformer, and individually self biased push pull 300B outputs.
It had No negative feedback, unless you count using a cathode coupled splitter and push pull interstage as having feedback. It legally does have Local negative feedback (but not thought of that way by most casual observers). The cathode to cathode, and plate to plate couplings both linearize the 6C45pi gain/splitter stage's even harmonics.
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I'm flattered that someone likes my website. I changed the name, but it's still up there. It's now: ephaseaudio.com
Here's my rendition
Power Supply -
Hammond 363CX power transformer
Separate filament transformer
1200V fullwave Hexfred B+ diodes
100 uf caps paralled with 0.1 uf per filter section
1.5H choke in C-L-C section
CL-130 inrush limiter
Fast recovery 1N4007 bias supply diodes
100 uf bias supply caps
Separate B+ switch
Separate bias pots per output tube
Driver/Power Section -
Triode Electronics driver circuit
EF86 pentode front end
12AU7 concertina phase inverter
Point to point wiring
Separate signal and power star grounds
Hammond 1650K OPT's
Multimeter test points for EL-34 bias
High quality metal film resistors
High quality poly-prop coupling caps
20 hz high pass input filter
Cosmetic -
Black anodized chassis
Hardwood side plates
Front panel toggle switches
Button head alloy screws
To do -
Measure and tune AC balance
Measure and tune GNFB and Zobel network caps
Power Supply -
Hammond 363CX power transformer
Separate filament transformer
1200V fullwave Hexfred B+ diodes
100 uf caps paralled with 0.1 uf per filter section
1.5H choke in C-L-C section
CL-130 inrush limiter
Fast recovery 1N4007 bias supply diodes
100 uf bias supply caps
Separate B+ switch
Separate bias pots per output tube
Driver/Power Section -
Triode Electronics driver circuit
EF86 pentode front end
12AU7 concertina phase inverter
Point to point wiring
Separate signal and power star grounds
Hammond 1650K OPT's
Multimeter test points for EL-34 bias
High quality metal film resistors
High quality poly-prop coupling caps
20 hz high pass input filter
Cosmetic -
Black anodized chassis
Hardwood side plates
Front panel toggle switches
Button head alloy screws
To do -
Measure and tune AC balance
Measure and tune GNFB and Zobel network caps
An externally hosted image should be here but it was not working when we last tested it.
Thanks, Bob. I read about your "musicbox" amp. I have a lot to learn from the calculations section. I can just about figure plate dissipation and add up the filiament currents. Nice point to point construction. You must live in Green Bank WV to not pick up AM radio with a wood chassis and no steel cover. Interesting theory about no-feedback and the harmonics it produces. If I want to quickly listen to a tube amp with no feedback, I could just wire mixer inputs to my spare Hammond H-100 amp. It's a bi-amp, 30 W 7591 bass, 7 W 6BQ5 treble (twice). Don't know if the output transformers are capable of anything above 7 khz though.
Hello EL34Dave.
I am currently building a ST70 clone using a pair of Hammond 1650k and i want to know how you hooked up them necause they are intended as parallel push-pull and using two tubes change the output impedance.
regards.
I am currently building a ST70 clone using a pair of Hammond 1650k and i want to know how you hooked up them necause they are intended as parallel push-pull and using two tubes change the output impedance.
regards.
About a year ago, I purchased (very cheaply I should say), a Chinese built Dynaco 70 clone that sounds really good (through my old, but excellent Mission 780s). Very quiet in operation, full sound stage, detailed with good dynamics etc. Apart from the heat output, I was perfectly happy with it. Then I happened to purchase, on Gumtree, a series 2 Creek CAS 4040 integrated amp and plugged that in instead. Well, it blew the tube clone out of the water! Much better dynamics, wider soundstage and just more power. This little amp cost me less than half the outlay for the tube amp, although admittedly, it's about 30 years old! Its soundstage and dynamics, however, are very tube-like, uncannily so in fact, and it may even be too 'rich' for some, but I just love it! Over this year's unusually hot summer (for the UK), it replaced the Chinese clone and maybe, just maybe, I may retire it for the winter, when I can justify powering the clone up to help keep the chill off the corner of the room where my system sits!! I keep asking myself how Mike Creek, then a very small independent London-based manufacturer, managed to produce such a stunningly good integrated amp for so little money back in the day. Today, his contemporary amps go for much higher prices (relatively) and garner great reviews all round. I must listen to one some day...
I'll be very interested to hear how you like those Hammond transformers compared to the original Dynacos. I built a 6L6 PP amp using Edcor's 6.6K primary trannies. They're okay. Nothing special, but they get the job done. The original design used Hammond 1650HA transformers, and in order to use the Edcors I had to use significantly less feedback. Square wave ringing was not particularly impressive.
My next amp will use Hammond 1650P transformers, but I likely won't get much done on it until December when I come home for winter break. People are always saying that the Dynaco transformers are great, but I've seen very little comparison between the Dynaco and Hammond transformers.
Sometime I'll have to get my hands on an ST70 so I can test them. It'd be very interesting to see how they stack up against the Hammond and Edcor offerings, and I'm dying to compare an ST70 to some of its original market competition: The Heathkit AA-121.
If and when I get one/have time, I'll be sure to post some test results.
Hi 100db,
I used the 1650K's because I got a good deal on them. The 1650N or 1650P would match up a little better with non-paralled EL34's, but I doubt if any one of us could tell the difference, even with our golden ears. Although you could try a different output tap in order to more closely match due to reflected impedance, but again, I doubt it would make any difference, sonicly (electrically, none).
I will tend to agree with you. I have a pair of Quicksilver mono blocks with 1650K output transformers on them and they don't sound any different than a pair of stock Quicksilvers. I haven't tried them on the Dynaco chassis as of yet but I seriously doubt there would be any sonic difference.
Hello, may ask, if I can use 1650HA output transformer (6k6/40W) for ST-70 please? I will use driver board with ECF82 probably, and 6L6GC for beginning. I think, that it will work, with lower power output.
Another question is, does anybody try russian 6f1p/6Ф1П (6BL8) in driver stage instead original 7199? Thanks.
Another question is, does anybody try russian 6f1p/6Ф1П (6BL8) in driver stage instead original 7199? Thanks.
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