Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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TheGimp
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Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by TheGimp »

Cryillic letters don't show in title so I changed it.

I owe a buddy of mine in Canada a favor and knowing he plays guitar I planned on making him an amp. I had already started a design when I found this site and decided a lite version of the Trainwreck Express (Liverpool) would be a nice gift.

I do my schematics in LTSpice, so the schematic is only the amp and does not have the power supply with it.

First pass is basic Expresso converted to 6N2P and 6P1P. I changed the output tube bias to cathode bias. Later I'll try fixed bias for class AB1. I'll build a prototype with cathode bias first. The resistor values arae wrong and need to be 220 Ohm.

Switching to the TW means the power transformer I have on hand does not have enough heater supply current so I'll have to order a new transformer. (building delay).
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Colossal
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by Colossal »

I noticed that you have 120k (R6) for your PI plate load. Is that intentional or did you mean 100k? Interesting that you inject NFB at the clipping stage.
mcrracer
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by mcrracer »

Thanks
TheGimp
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by TheGimp »

The 120K was set in place of a 100K when the simulation showed an imbalance in the output of the two sections of the phase splitter. I suspect a simulation error but am not sure exactly what is wrong.

Thanks on the fb. It is connected wrong (early am this morning/last night). I'll move it.
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VacuumVoodoo
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by VacuumVoodoo »

6P1P is a pretty cool tube. One could even call it a little sister to 6V6 in a 9 pin noval bottle. An Raa=8k OT and Ua=325V gives ca 10W clean before clipping.
The amp in these clips: http://www.youtube.com/olekelo
runs power amp with 2x6P1P per above Raa & B+. No TW DNA in the circuit.
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dartanion
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by dartanion »

The PI imbalance is purposeful.
Eardrums!!! We don't need no stinkin' eardrums!
TheGimp
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by TheGimp »

I could understand a little imbalance, but this is 6:1

I've never seen this much imbalance in an LTP. I guess the only thing to do is throw one together and see what it does.

I found I have an old 6V6 output transformer in my parts stash, along with a Stancore PT8401 that is about minimum for building an amp.

If I can get the pots locally, I think I have the rest of the parts such that I can throw something together in time for the race the end of this month.

Does it matter in a guitar amp if I use UF4007s instead of 1N4007 for the rectifiers? I switched when I had problems with a stereo amp that had buzzing in one channel with 1N4007 and it went away with UF4007s. I understand the 1N4007 should be used in the output circuit.
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TheGimp
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by TheGimp »

I've read from page 60 back to the begining, and a good bit from here back but still haven't found some answers about the phase splitter.

However a little analysis and I think I understand what is going on.

The disparity in output from the LTP is intentional and is caused by the feedback. It is not Local Negative Feedback as I had originally guessed, but rather Local Negative/Positive Feedback. One half of the LTP sees Negative Feedback while the opposite half sees Positive Feedback.

The greater the drive, the greater the disparity of the output from the two sections. This results in an amp that is producing rapidly increasing amounts distortion up to the point where one output starts to go into saturation and cutoff. The other output is still linear but receiving positive feedback which has now resulted in additional odd harmonic distortion.

Obviously this is intentional and what Ken wanted to produce a profile that was clean at low playing levels with rapidly increasing distortion as drive went up.

The only issue I can see is whether feedback scaling is again necessary when considering smaller tubes with lesser output.

From Mullard Jan 1964
EL34 data sheet (Page D2) for two tubes in PUSH PULL fixed bias (no U.L.) specifies :
“Vin(g1-g1)r.m.s. 50 V”
EL84 data sheet (Page D1) for two tubes in Push Pull fixed bias pentode connection specifies:
“Vin(g1-g1) r.m.s. 20V”

So EL84s only take 40% of the drive voltage necessary to drive EL34s. Based on this it seems to me that properly scale the Express front end to drive a quad of EL84 in the Liverpool, the last gain stage prior to the phase splitter should have it’s gain scaled down to 40% of the Express stage.

Or better yet, scale the output of the phase splitter (replace each anode resistor with two resistors) to provide proper level scaling without effecting distortion profile.

No Log (audio) Taper Pots available at the local electronic store. No good selection at Radio Shack (100K Alps was the only good pot). Looks like I have to order parts from AES.
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VacuumVoodoo
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by VacuumVoodoo »

You moved NFB back one stage so you changed internal phase relationships. Swap connections of output transformer primaries.
Your old presence pot and cap are still in the schematic but will have no effect now, they are inside NFB loop, not a direct part of it.
Get rid of the 5k pot and put a 47k resistor there instead.
C14: connect the low end to ground instead.

This has a chance of curing your headache.
Aleksander Niemand
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TheGimp
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by TheGimp »

I think I made all the changes before the lasts simulation.

I also found the FB resistor was 1000K not 100K and changed that to agree with the Express and Liverpool schematics.

If feedback is to the cathode of a LTP from the output transformer secondary, it can not be NFB for both triodes. It has to be NFB for one and PFB for the other. This will throw the AC balance off.

This schematic also has the resistor dividers to scale the drive to the output tubes.
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VacuumVoodoo
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by VacuumVoodoo »

TheGimp wrote: If feedback is to the cathode of a LTP from the output transformer secondary, it can not be NFB for both triodes. It has to be NFB for one and PFB for the other. This will throw the AC balance off.
It is NFB around the amplifier including the PI. Note that right side triode has its grid grounded for AC.
Perhaps you should read up on how LT PI works, http://www.aikenamps.com/LongTailPair.htm is a good starting point.

The imbalance you're getting I've seen only once when I plugged an ECC832 (12DW7) in the PI socket by mistake.
Aleksander Niemand
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mcrracer
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by mcrracer »

So is the NFB as drawn correct or wrong? Should I just implement NFB per the Liverpool schematic? Thanks
TheGimp
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by TheGimp »

It seems to me that the Aikenamps site says the same thing I am saying:

"The third input is not so obvious; it is the lower end of R6. If a signal is input at this point, the phase splitter will produce an output signal on each output that is in phase with the other, rather than 180 degrees out of phase, and also in phase with the signal input at the lower end of R6. This means that if a signal of equal phase is applied to the first input (C1) and the third input (R6), it will subtract from the out of phase output (R2) and add to the in phase output (R1). Likewise, if an equal phase signal is applied to the second input (C2), and the third input (R6), it will subtract from the in phase output and add to the out of phase output..."
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VacuumVoodoo
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by VacuumVoodoo »

It's all very nice, smart and pretty but conduct a mental experiment with the signal applied to the "third input" being taken from amplifiers output i.e. THE NFB signal. Go through the same mental torture with swapped primary connections on the OT. Something happens as if by magic.....
Aleksander Niemand
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TheGimp
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Re: Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

Post by TheGimp »

Ah, thank you Aleksander.

In goign thought the mental exercise I found I had left out resistor R39 which made a profound difference in the way the circuit operates.

I also corrected a few incorrect values.

I have re-assigned component designators to make my schematic consistent with the 451 schematic and verified my schematic so hopefully I will have no more such confusion.
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