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Old 20th March 2006, 02:39 AM   #1
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Default Suggestions for Upgrade - VDV70/100

I have a pair of PPP EL34 monoblocks built to the design below.
I have the EL34s Triode Strapped with the screen resistors (R14a,R14b, R15a, R15b) = 1K 5W. With an Oz 250V 50Hz AC Line I get an HT of 485 V. I also run ECC99 in place of the 12AU7 with no other circuit changes.

Problems are:
Gain too Low
Zout too High

Otherwise sound is surprising good.

Those VDV2100 (PAT4006) Toroidal Outputs really deserve something a bit better, particularly in the front end / driver. I have a few ideas about a rebuild BUT though I'd canvas the "experts" before diving in.

Any suggestions from the experts ("drips under pressure") here?

Cheers,
Ian
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Old 20th March 2006, 03:47 AM   #2
lndm is offline lndm  Australia
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Bypassing your output stage cathode resistors should lower your output impedance.

Would that subsequent increase in gain then allow you to do away with the first stage, bias the splitter using resistors and capacitor couple it? or use a long tail phase splitter instead?
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Old 20th March 2006, 05:19 AM   #3
lndm is offline lndm  Australia
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Oops! didn't read that right. Disregard the second paragraph. The bypass cap will increase the gain and lower the Zo.
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Old 20th March 2006, 05:29 AM   #4
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Indm,
Those cathode Resistors are 10 Ohm. That is the amp is fixed biased and these are just convenient bias measuring resistors and individual tube fuses. Bypassing those resistors WILL increase gain and lower Zout - by about 0.5%.

Actually I was thinking in terms of a whole new front end/driver. The Output Transformer bandwidth is so huge that global feedback does not worry me BUT I am hoping someone might have a good idea for local feedback schemes which eliminate or at least reduce the requirement for any global feedback.

Perhaps a separate pentode driver tube (or triode with largish unbypassed cathode resistors) or even MOSFETs for each output tube with shunt ("partial") feedback.

Cheers,
Ian
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Old 20th March 2006, 06:05 AM   #5
lndm is offline lndm  Australia
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Ahh, the best laid plans go awry sometimes.

I have found whilst measuring and comparing output taps, that damping factor can be increased by putting an 8ohm speaker on the 4 ohm tap. Sometimes neither of the two taps is perfectly ideal and you can choose one over the other.

I was just talking elsewhere about the 6dj8. I was told that it can be used for a gain of 25
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Old 21st March 2006, 02:52 AM   #6
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Can anyone comment on pros / cons of differential mu stage front end / driver as compared to diff amp front end feeding SRPP drivers?

Should I instead be looking at interstage transformer solutions?

Cheers,
Ian
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Old 21st March 2006, 07:50 PM   #7
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Ian,
Take a look at Norman Koren's web site. He has some interesting input phase splitter design. The web site is www.normankoren.com

Andy
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Old 22nd March 2006, 03:21 AM   #8
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Andy my friend, give me a call .
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