Converting Yarland FV-34AIII to Triode and Zero NF - diyAudio
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Old 10th March 2009, 11:07 AM   #1
jolon is offline jolon  Australia
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Question Converting Yarland FV-34AIII to Triode and Zero NF

I purchased a Yarland FV-34AIII amplifier a few weeks ago which is my first introduction to valve amps. I got it fairly cheap and it was advertised as SET and Zero Negative Feedback. I was suspicious from the start whether it really was SET as I had read on a few forums that people had converted their FV-34AIII to triode, so I'm presuming it must've been pentode to begin with!

I found this schematic for the 34A:

http://www.petemoore.pwp.blueyonder....udio/FV34A.jpg

Not knowing much about valve amps I'm presuming that the schematic indicates Pentode configuration with negative feedback.

I decided to open the amp up and have a look and I think I can pretty much confirm that it is Pentode with Negative Feedback. However, what is interesting is that there doesn't appear to be any resistor between pin 4 of the EL34 and the output transformer (such as the 200 Ohm in the schematic). All of the SET schematics I have seen have a resistor there. I'm interested to know what people think about there being no resistor there?

There are many differences between the schematic and my amp (possibly because it is now up to version III). For example there is no 22uF capacitor connected to pin 4 even though there is a space for it on the PCB.

My plan is to cut the tracks of the PCB that connect pin 4 to the OPT and the 280V supply and solder a 200 Ohm 2W resistor directly between pins 3 and 4 of EL34. I was wondering if this seems okay and whether 200 Ohms is the best value to use? I noticed in another schematic that 100 Ohms is used:

http://webpages.charter.net/porkchop...0schematic.jpg

I'm also interested to know what brand resistor would be the best to use?

The negative feedback wires seem to be soldered directly to the speaker terminals which then come back to the PCB. They are brown in colour, does that sound right?

I'm fairly new to DIY audio so any help is very much appreciated.
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Old 10th March 2009, 02:06 PM   #2
45 is offline 45  Italy
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For triode strapping, use a 150 or 220 ohms flameproof resistor. 2-3W power rating.

Like these:
http://www.vishay.com/docs/31021/cpf.pdf

A SE triode-connected EL34 is not very linear, typically. Before removing irreversibly the feedback connections I would check if it is all right without. Sometimes with little amount of Fb it is simply better!

45
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Old 12th March 2009, 02:03 PM   #3
jolon is offline jolon  Australia
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Excellent, thanks for your help.

I've ordered 200 ohm 2W, 220 ohm 2W, and 100 ohm 3W, of the resistor you linked to (Vishay CPF).

Will probably use the 200 ohm 2W, if only because it was in the original Yarland schematic.

Thanks also for the advice about negative feedback.
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Old 23rd March 2009, 12:50 PM   #4
jolon is offline jolon  Australia
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Well... I converted the Yarland to triode strapped using two 100 ohm 3W resistors in series and... the sound was quite unimpressive.

It lost the nice EL34 tone.

In pentode mode the amp was a bit harsh in the high end, in triode mode the overall sound was much more subdued, but a little dead, the nice EL34 midrange was gone. The bass was a little heavier though.

I thought maybe I'd done something wrong with the PCB cutting etc, but rewired it back to pentode and that beautiful sound has been restored.

So thanks for the warning about EL34 in triode mode!

After reading about the benefits of triode mode I thought the EL34 would sound better, so was quite surprised.

After converting to triode mode, my father brought down his newly acquired Yaqin 100B (KT88 push-pull) and it sounded really beautiful in both ultralinear and triode modes. Which makes me think that it is just the EL34s that don't sound good in triode mode.

Will stay with pentode mode for now and maybe try some KT88s down the track.

btw - I didn't end up removing negative feedback.

Thanks again for your help.
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Old 11th January 2012, 04:11 PM   #5
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Default How to Bias the Yarland FV-34A tube amp?

Dear Jolon and anyone else who owns the Yarland FV-34A tube amp,

HELP!

I just received the Yarland FV34A tube amp today.

Unfortunately, the English manual does not explain (or rather it tries to explain but fails utterly) on how to bias this amp.
If you happen to own this amp, please share with me on how to carry out the proper steps to bias this amp.

Thanks!
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