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Audio Institute VR-70E amp
Audio Institute VR-70E amp
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Old 24th December 2017, 07:25 AM   #11
artosalo is offline artosalo  Finland
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There seem to be factory built unbalanced cathodyne. Ra = 33k, Rk = 36k ???
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Old 24th December 2017, 07:28 AM   #12
petertub is offline petertub  Sweden
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Quote:
Originally Posted by artosalo View Post
There seem to be factory built unbalanced cathodyne. Ra = 33k, Rk = 36k ???
Yes, this error is shown in all published schematics.
I have used and given away one of these, but i never looked inside, and had
no schematics at the time.
I guess that the feedback will compensate for the unbalance but yes it should be
corrected.
Funny that the anode resistors are located on the powersupply board !
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Old 26th December 2017, 05:19 PM   #13
mintymiller is offline mintymiller
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Quote:
Originally Posted by jazbo8 View Post
What’s the rationale for the derating? To get high-power class aB output?
No idea why it was derated even if it has actually been done. I would have thought the easiest way to ‘derate’ would be to drop the bias current but that seemed relatively normal to me for an EL34 with ~450v on the anode plates although the measured 520 mV is a bit high for 38 mA current.
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Old 26th December 2017, 05:35 PM   #14
petertub is offline petertub  Sweden
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I'm only speculating here ;
If it's "derated" the cheapest and simplest way is to wind smaller output transformers
with smaller diameter wire.
Anyone has a derated and a VR70 to compare with?
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Old 26th December 2017, 05:58 PM   #15
mintymiller is offline mintymiller
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Thanks for the tip! I’ll try that but having set it to 38mA, which with my amp gives ~450mV suggests the cathode R is ~10 ohms. Should running in Class A for much of the time.
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Originally Posted by petertub View Post
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 26th December 2017, 06:01 PM   #16
mintymiller is offline mintymiller
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I think it highly unlikely that anyone would bother doing that.
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Originally Posted by petertub View Post
I'm only speculating here ;
If it's "derated" the cheapest and simplest way is to wind smaller output transformers
with smaller diameter wire.
Anyone has a derated and a VR70 to compare with?
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Old 26th December 2017, 09:24 PM   #17
petertub is offline petertub  Sweden
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Originally Posted by mintymiller View Post
I think it highly unlikely that anyone would bother doing that.
Any producer that tries to cut cost could do this. I'm talking about new production of an amp, supplying with cheaper transformers does make sens
if a reduction of performance is accepted.

But we have no real info if this reduction is actually done.
Measuring power output at 1000hz would give a clue, my rememberence of vr70
is that they produced 35w/channel just liek one would expect.
If power measurment is not possible, measuring the B+ ( or the plate
voltage on one EL34 at idle ) could be compared with the schematics. Rewinding the mains transformer to supply lower voltage is possible, but
the savings are only half of the saving cheaper output transformers will give.
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Old 27th December 2017, 01:37 AM   #18
PRR is offline PRR  United States
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Quote:
Originally Posted by mintymiller View Post
..same amp supposedly derated to 20W
...9mA to 51mA so after adjustment, I set them all around 40mA (~520mV)...
The idle current has very little to do with the full power output in a push-pull amp, especially fix-bias.

I wonder if the "derating" means a higher load impedance for more linearity. Or just "rating" at a lower point on the rising THD curve. The same amp can be 1%THD at 40W or <0.4%THD at 20W. If the market favors low THD number rather than high Watt numbers, a maker could change the ad-sheet specification with no change in the amplifier.

When in doubt, it is rarely wrong to bias a decently-designed fix-bias pentode(/UL) amp around 70% of maximum plate dissipation. There is no "magic number" here, just a broad minimum which for "most" amps includes 70% Pdiss as a good point.

Taking the conservative 25W Pdiss number for EL34, we aim for 17W-18W. If plate voltage is 420, we want 40mA-43mA plate current. It is far safer to measure cathode current. This includes screen current. On EL34 at typical idle, Ig2 is about 15% of Ip. So we get a number like 48mA cathode current for "70% Pdiss".

I'd take 40mA or 52mA. I would not expect a huge difference. I'd like match to 20% or 10mA, but I have seen quite "poor" matches work very fine.

Using old-school 10 Ohm cathode resistor, this is indeed 400mV-520mV.
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Old 31st December 2017, 06:36 PM   #19
mintymiller is offline mintymiller
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Actually, I assumed a plate voltage of 450 based on previous owner’s notes and I based my calculations for bias current on the 70% rule of thumb before following Weber’s guide. I was happy to note that the bias current I’d calculated was similar to that noted for a 450V plate voltage running at 70%.
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Old 2nd January 2018, 07:33 PM   #20
tricomp is offline tricomp  Denmark
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Quote:
Originally Posted by petertub View Post
Yes, this error is shown in all published schematics.
I have used and given away one of these, but i never looked inside, and had
no schematics at the time.
I guess that the feedback will compensate for the unbalance but yes it should be
corrected.
Funny that the anode resistors are located on the powersupply board !
Well, I'm the one who made the schematic in posting #10 and it hasn't got an error. The resistors in the split-load phase-splitter are NOT equal in the actual amplifier. Maybe they SHOULD be, but they're not.
The schematic came about this way:
I saw the amplifier VR 70e-2 advertized in Germany for an attractive price. I didn't want to invest in something I didn't knew how was built so I asked the shop for the schematic diagram. I didn't have high hopes for ever getting a response but in a couple of days the schematic was in my e-mail.
I have included here what I received and you will easily recognice the typical and flawed Chinese-style diagram with sloppy errors. I was disappointed but still I ordered the amplifier.
The amplifier arrived and I immediately took it apart. I traced all components and made the schematic diagram you've already seen. The anode-resistor of the P/I isn't 'in the power-suppy section' as suggested. From the original schematic you'll see it has just been drawn like it belongs in another section of the amplifier. It doesn't, as there is only one PCB carrying all components except for a small add-on board with extra powersupply filtering. This extra board btw, is what mainly sets the VR-70e-2 apart from the original VR-70e model.
The amplifier has seen much use since it was bought in 2011 and still carries the original tube-selection. Actually today I compaired it with a NAD-214 on my set with B&W DM610-S3 speakers. The NAD had a harsh, typical transistor-related aggressive sound. The VR-70e-2 has nothing of the sort. It's an amplifier you may listen to for hours and hours without tiring of the sound.
Attached Files
File Type: pdf VR-70E-2_Fax-copy_.pdf (115.6 KB, 65 views)
File Type: pdf VR-70e-2 Photos.pdf (971.9 KB, 52 views)
File Type: pdf VR70E 2_sch.pdf (46.4 KB, 71 views)
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