Hi,
I checked thru past posts and couldn't find anything on the Allen 75 amp except a thread asking if the amp was a mono or stereo and or if it could be converted to a stereo amp. The other posts are related to selling the amplifiers in the swap meet section.
I am looking for suggestions for modifications to the amplifiers to make these very good sounding amps even better.
So far I have looked over the schematic on the amp and I see there is a input amplification stage a 12Ay7 that isn't needed because it not only amplifies the signal too much but in the process injects a lot of noise.
I'm not thrilled with the EF86 tube which looks to me to be the phase splitter section. I've seen that others have changed this tube to a basic 6sn7 type 9 pin tube but this makes me wonder about the miller effect.
Does anyone have any suggestions? Leave the EF86? Change it to something else? Pull it out and use something else?
Thanks,
Joe
I checked thru past posts and couldn't find anything on the Allen 75 amp except a thread asking if the amp was a mono or stereo and or if it could be converted to a stereo amp. The other posts are related to selling the amplifiers in the swap meet section.
I am looking for suggestions for modifications to the amplifiers to make these very good sounding amps even better.
So far I have looked over the schematic on the amp and I see there is a input amplification stage a 12Ay7 that isn't needed because it not only amplifies the signal too much but in the process injects a lot of noise.
I'm not thrilled with the EF86 tube which looks to me to be the phase splitter section. I've seen that others have changed this tube to a basic 6sn7 type 9 pin tube but this makes me wonder about the miller effect.
Does anyone have any suggestions? Leave the EF86? Change it to something else? Pull it out and use something else?
Thanks,
Joe
There is no need for good treble response in an organ. But bass response is important - so the output trasformer might be optimised to provide such a response.
However, in phase splitter circuits, miller effect is gnerally neglible. If the phase splitter is an EF86, that would be an unusuall choice, but indicates it is probably wired as a concertina phase spitter. This mandates operating in triode mode (screen strapped to anode), and the voltage gain is unity. With unity gain miller effect is almost non-existent.
However, in phase splitter circuits, miller effect is gnerally neglible. If the phase splitter is an EF86, that would be an unusuall choice, but indicates it is probably wired as a concertina phase spitter. This mandates operating in triode mode (screen strapped to anode), and the voltage gain is unity. With unity gain miller effect is almost non-existent.
Why not post the schematic? Since it is apparently not available on this site other owners can benefit from having it; further, those who are technically inclined can offer suggestions as to how to improve the circuit once they know what it is.
Well, the pahse spiltter is a pentode and it is a concertina. But the screen is bypassed to cathode, so it will work just fine.
There is no need and no benefit to change the pentode for a triode.
It is a surprisingly good amp - UL output, phase splitting done at low level.
There is no need and no benefit to change the pentode for a triode.
It is a surprisingly good amp - UL output, phase splitting done at low level.
The EF86 is a voltage amplifier, followed by a Mullard-type phase-splitter. Why do you want to replace it? Have you tried running the amp with V1 bypassed as others have suggested?
Ok, already made up my mind to bypass the 1st stage which is a 12Au7 because this stage simply isn't needed and contributes to too much gain and noise.
Does anyone have any ideas how to hot rod this amp? I posted a schematic in a prior post.
Does anyone have any ideas how to hot rod this amp? I posted a schematic in a prior post.
I believe you're misreading the schematic. No concertina. The pentode is a simple VA with NFB fed into the cathode, then a LTP phase splitter, then CF feeding the power tubes.
I will agree with you the pentode is a VA and the first 6SN7 is a long tail pair and the second 6Sn7 has to be a cathode follower.
Is it possible or maybe better put is there any advantage to modifying the circuit and inserting a 6SL7 as the LTP instead of the 6SN7?
Is it possible or maybe better put is there any advantage to modifying the circuit and inserting a 6SL7 as the LTP instead of the 6SN7?
What is the value of the bias circuit as built? Never come across this particular bias circuit.
Advantages?
Advantages?
Advantage for what? SL7 will produce higher gain. You need to devise your gain structure strategy. Do you need or want more gain? What are your signal levels, or what do you want them to be, at each stage? Will you have more signal at the SL7 input than the SL7 can handle?
Don't understand the question. Which bias circuit? Each tube is biased. There is no biasing in that amp that's rare or unusual.
I've only seen the bias after the phase splitter coupling caps not going to the grids of the cathode follower.
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Again, I don't understand your question. Try to be more precise.
If you're talking about the 2 pots, that's pretty normal. R28 adjusts the overall bias for both power tubes, R26 equalizes the bias between the 2 sides.
If you're talking about the 2 pots, that's pretty normal. R28 adjusts the overall bias for both power tubes, R26 equalizes the bias between the 2 sides.
take a look at a dynaco OR QUICKSILVER amplifier and you will a difference in the way the bias is introduced. You will understand my question then.
None of those have cathode followers. They have concertinas AC coupled to the power tubes directly. So no, they wouldn't be anything like your amp.
Take a look at ARC D90.
Take a look at ARC D90.
Joe
I just noticed your thread but I have rebuilt many allen 75s and I have eight moire here I'm doing for people.
The best thing to do on these amps is to take out the 12ay7 preamp module and put in its place a 100uf solen for a filter cap..You need to change the royalite caps out underneath and put in a pair of 820uf at 250v panaonics..Yes,they are a lower voltage than the original but they work perfectly as they have a 325 surge and they are in series.
You want to put a film cap power supply in most of the amp and I have a whole list changes I made for the feedback loop to make it conducive to hifi.
Here is an older one I did but I made more changes since.
pics of the allen 75s - Michael Samra - Tube DIY Asylum
Thanks for the reply. Can I possibly contact you for a little more info? I have bypassed the 12Ay7 stage. I have new coupling caps coming to replace the factory ones. Are you talking motor run filter caps?
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