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Audio Institute VR-70E amp
Audio Institute VR-70E amp
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Old 24th August 2014, 09:26 AM   #1
batteryman is offline batteryman  United Kingdom
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Default Audio Institute VR-70E amp

A few years ago I bought a budget VR70e from a UK Ebayer. Apparently derated from 40 to 20watts to justify the low price (or poor quality EL34s?)

One EL34 has failed, but I am confused by the test point voltages. (test points are connected to the cathode)
The working channel's are both around 35mv but according to some other websites, should be 10 times this. The cathode resistors are 10R so the current is 3.5ma. The anode voltage is 450, so this means the valve is only dissipating less than 2 watts.
Or am I missing something?

I am intending to replace the 6F2s with ECF82s and fit a matched set of 4 EL34s as well as replace the output stage coupling caps with better ones. (The input caps I already changed to some Audio Note ones I had left from another project)
Ian
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Old 24th August 2014, 03:58 PM   #2
jazbo8 is offline jazbo8
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Audio Institute VR-70E amp
35ma x 10 = 350 mV, and why do you need matched set when you can adjust the bias of each tube individually?
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Old 24th August 2014, 04:42 PM   #3
batteryman is offline batteryman  United Kingdom
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Yes I used my supercomputer to work out that the voltage across the cathode resistor should be 350mv, that's why I wondered why it was only 35mv!

It cost the same for a matched set of four, as individual non matched ones and I believe there is more to it than just being able to adjust the bias for equal current, such as gain, MU, transconductance GM & plate resistance Rp.

This formula seems to connect the 3 parameters than should be matched for triode connection:
Mu = Gm x Rp

I am happy to put right on this, being at the bottom of the learning curve.
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Old 24th August 2014, 05:01 PM   #4
jazbo8 is offline jazbo8
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3.5mA would suggest the tube is shot or something is wrong with its bias voltage. You are right about the matching parameters, but how many sellers would actually do it (without some decent markup), and how would you verify it without the proper test gear at home... And of course, to use a matched set, you should have OPTs with perfectly matched primary windings, what is the likelyhood that such transformers are included in an amplifier at this price point?
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Old 24th August 2014, 05:26 PM   #5
batteryman is offline batteryman  United Kingdom
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I agree with your points and of course it means taking the seller on trust and that's something you cannot take for granted from internet businesses, sadly.

Also, surely the standing current mainly determines at what point the output changes from pure class A to AB? For sensitive speakers and / or low volume the amp should sound better if the standing current is high enough to keep it running in class A.
But if the speakers are not sensitive or always run quite loud, then it will be running in AB so could the standing current can be lower without affecting the sound?
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Old 24th August 2014, 07:23 PM   #6
jazbo8 is offline jazbo8
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Yes, you can vary the bias of the output stage to suit your needs, in addition with the ultralinear OPT, you can also try out different output tube connections.
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Old 23rd December 2017, 05:36 PM   #7
mintymiller is offline mintymiller
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Default Re biasing this amp

I think I have the exact same amp supposedly derated to 20W per channel, sold on to me a few months ago through another eBayer. Considering how cheap it was, it sounds very nice. Having read batterymanís experience from three years ago, I recently checked the bias voltages and current. Iím pretty sure it has some recently renewed EL34s so a check was due. To be (down) rated to 20W, I expected the settings to be set around 24mA. Before adjustment, they varied (valve to valve) from 39mA to 51mA so after adjustment, I set them all around 40mA (~520mV). Took some patience as adjustment took a while to settle down. I havenít checked, but from schematics of the Dynavox version, Iím assuming the valve HT is around 420v, so that gives me around 34W per channel, well within EL34 tolerance and more than adequate for my listening. Itís a very decent amp, just missing its cage. Iíd be grateful If anyone knows where I might get one. The guy who runs the company that sold this unit to its first purchaser is unlikely to sell me one! Iíll be taking it apart in the summer to mod for switchable triode connection. Thanks
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Old 24th December 2017, 06:05 AM   #8
jazbo8 is offline jazbo8
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What’s the rationale for the derating? To get high-power class aB output?
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Old 24th December 2017, 06:32 AM   #9
petertub is offline petertub  Sweden
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Schematics please !
As regards to the low bias measured ( 35mV) ; measuring the cathode resistors will
answer what bias voltage ought to be. The cathode is connected to pin 8 in the socket,
the cathode resistor is connected between pin 8 and ground.
To measure, amp disconnected and cooled down, lift the EL34, connect the DVM between
pin 8 and chassies ( rivet that fastens the socket will do) and read the resistance.
If 1 ohm is then 35mV is reasonable as bias , 10ohm then 350mV is ok
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Old 24th December 2017, 06:36 AM   #10
petertub is offline petertub  Sweden
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I seem to have a schematic :
Attached Files
File Type: pdf VR70E 2.pdf (305.8 KB, 192 views)
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