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Old 28th November 2018, 02:37 AM   #311
TheGimp is offline TheGimp  United States
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Originally Posted by dch53 View Post
For global negative feedback George recommended adding a 100Ω resistor between the 0.47uF capacitor and ground and running the negative feedback via a 5k resistor to the junction of the capacitor and resistor.

In my simulations I've run a 2.5K feedback resistor without a problem. Gave me about 10dB of feedback I think.
This isn't clear. Does the new 100R resistor go in parallel with C1, and the feedback go through the 5 resistor to the junction of C1, R8, the new 100R resistor and the pot output?
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Last edited by TheGimp; 28th November 2018 at 02:46 AM.
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Old 28th November 2018, 03:09 AM   #312
Tubelab_com is offline Tubelab_com  United States
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No, lift the ground end of C1 out of the board and wire a 100 ohm resistor in series with the cap. The resistor goes between the ground hole in the board and the lead on the capacitor that you just pulled out of that hole. Then inject the feedback into the junction of the cap and resistor. This is done with as resistor in the 4000 to 10,000 ohm range from the speaker terminal. Most OPT's will also work better with a small cap across this resistor.

If a phasing issue causes the amp to oscillate, swap the plate leads on the OPT.
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Old 28th November 2018, 03:20 AM   #313
TheGimp is offline TheGimp  United States
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Thanks for the clarification George. That makes sense as the parallel would impact the balance adjustment.

I already swapped my grid drives to correct the phasing so it should not oscillate.
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Old 3rd December 2018, 01:08 AM   #314
TheGimp is offline TheGimp  United States
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I think I'm running into AB2 territory as Vgk is +10V peak.

I'm getting 26W out with a 5K opt, 400V B+ with a pair of Russian/Soviet erra 6P3S.

2nd harmonic exceeds 3rd, but 5th is above both.

THD is 2.6% going into clipping on the positive peak at the OPT (8R load). I suspect one tube has much higher Gm than the other as clipping is asymmetrical.
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Old 4th December 2018, 03:18 AM   #315
TheGimp is offline TheGimp  United States
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Oops, blew up my sound card. I was getting 40+ W out at 450V B+ and had the resistor divider string to the sound card in the wrong position.

Looks like I'll buy another cheap sound card off amazon.

I think I will rework my wiring to the computer and bury a resistor and zener clamp circuit in series with the audio input to form a protection network for the future.
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Old 10th December 2018, 03:34 PM   #316
Tubelab_com is offline Tubelab_com  United States
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All of the current batch of these boards are gone.


Another batch will be ordered as soon as funds are available. The issue with my cloned debit card and emptied bank account has been fixed, but I'm still waiting on a large check.

There are two possible paths here:

1) Order another batch of boards that are identical to these. This is the quickest and cheapest option, and involves zero risk.

2) Make any desired changes and resubmit the board. This requires layout changes and generating new artwork files. Any board changes involve risk, and additional tooling charges. To mitigate the risk, I always spin a DIY test board, populate it and test it. This adds considerable time to the job, which is in short supply right now. It also adds a $100 tooling charge, which gets adds another $4 to the board cost for a 25 board purchase.

As of now the only change I would make is to add a second input connector so the GNFB or differential inputs could be used without lifting a capacitor lead.

My current choice is to leave the board alone and order more as soon as funds arrive. Any thoughts?

I'm currently busy with a contract job, and my available Tubelab time needs to go toward the redesign of the TSE. A new thread will be started for that subject, and it will need a lot of attention.
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Old 10th December 2018, 04:42 PM   #317
wanders is offline wanders  United States
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Originally Posted by Tubelab_com View Post
All of the current batch of these boards are gone.


Another batch will be ordered as soon as funds are available. The issue with my cloned debit card and emptied bank account has been fixed, but I'm still waiting on a large check.

There are two possible paths here:

1) Order another batch of boards that are identical to these. This is the quickest and cheapest option, and involves zero risk.

2) Make any desired changes and resubmit the board. This requires layout changes and generating new artwork files. Any board changes involve risk, and additional tooling charges.
FWIW, my vote is #1. I've spent a fair amount of time, energy, and $ to produce what I, and others, believe is a commendable set of amps. As more builders take on the existing UDB, more knowledge will be forthcoming. Like the old dog said "lifting a leg is all in a day's work."

Seasons greetings to all.'

Warren
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Old 10th December 2018, 10:44 PM   #318
dch53 is offline dch53  Australia
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There are two possible paths here:

1) Order another batch of boards that are identical to these. This is the quickest and cheapest option, and involves zero risk.
2) Make any desired changes and resubmit the board.
If you go for option 2 provision for elevating the heater of V2 would be good.

Unfortunately that would require an additional terminal block for which there isn't any room.
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Old 11th December 2018, 02:21 AM   #319
Tubelab_com is offline Tubelab_com  United States
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provision for elevating the heater of V2 would be good.
As you noted getting another terminal block in there would mean taking one out, or making the board bigger. Either would mean a total re-layout. I usually elevate the heaters for both tubes to 50 - 70 volts for most cases. That puts the heater 50 to 70 volts more positive that the cathode of V1, which is within spec, and well below 200 volts more negative than the cathode of V2 (in spec) even with signal swing. This assumes a 6CG7 or 6FQ7 for V2. It's worth a check and possible voltage change if other tubes are used.
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Old 14th December 2018, 03:46 AM   #320
Tubelab_com is offline Tubelab_com  United States
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I ordered 50 more boards identical to these. They should arrive in early January.
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