Express EL34 versus 6V6 + NFB Questions

[hans-jörg]

Hello,
I´m still hasitating to swap a 6V6 in instead the EL34.
Plate is 400 VDC, Screens are around 390 VDC. Bias at 40mA.

Can I put the 6V6 in without to rebias them as I read somewhere here? I can´t beileve. If I have to bias the 6V6 I´m not shure if I can get into the propper range, although I have a 50k pot there and the range is quite broad.

(I´ve switched once my D*stile 102 from 6L6 to EL34. Had to change some resistors to reach the right range.)

But the TW schems showing only EL34/6V6.

Could someone switch on my light?

Best

Hans-Jörg

[Gibsonman63]

I can't explain it in a technical sense, but I did run some NOS 6V6s in my Express Clone for about a year without rebiasing and with no ill effect. I had intended to do this as an experiment and check the bias, but somehow I got busy playing instead of tinkering with amps. (Sometimes, I need to satisfy the other half of my brain!)

My impressions of the 6V6 is that they had a little less clean headroom and volume. A good choice if you need to drop the power slightly and still get plenty of drive. To me, the Express starts to lose some of the magic if you turn down the volume pot on the amp too much.

I am now using EL34s again with an airbrake and find that it gives me more flexibility. More power when I need it and cleaner cleans when I want it.

If you are worried about the plate voltage, try some JJs.

[hans-jörg]

Hi,
the JJ are in my orderlist.
So if I understand right: the headroom goes a bit down, earlier brake up but sound like a Tweed de Luxe on stereoide?

Thank you

Hans-Jörg

[Gibsonman63]

I would say that's a pretty good description. At least that's how my amplifier behaved through my speakers and guitar.

[Reeltarded]

I have a pair of the JJs that happily nip along at the heals of Hell when directly swapped with a pair of EL34 RFTs that bias at 38.8 with 451v plates in a Marshall thing.

The amp loses the volume that makes your ears tickle when your nose is against the grille cloth, and it sounds more like a fist punch instead of being hammered into the floor.

[cbass]

You can still find NOS and used 6V6gt's for cheap. Any decent 6v6 should have no problem handling 400 volts.

I've experimented with running 480 on the plates with several old sets and they seemed fine after playing the amp several hours at a time.

[statorvane]

You should have no problem swapping 6V6 / EL34s. I rebias (JMP50 clone), but I'm sure you'll have plenty of bias trim available without having to heat up your iron, etc.

[hans-jörg]

Hi gents,

thank you for calming. So i´ll try the JJ I ordered.

Btw: I´m very proud on my amp. I didnt install a presence pot but conected the PI tail (10k) via 100k with the 8 Ohm tab. There is a PPIMV and a Cut control. But what I want to tell is: befor I made the Feedback connection I had a very hissi express clone. No humm but hissy. I saw no possibilities to stop this exept of a FB. And right, the amp is death quiet. So quiet evan at gain 12 o´clock that I had to put my ear at the grillcloat to hear if its working or not.
Thats a sucsessfull built for a high gain "monster". The Liverpool was much more noisy.
On the other hand it leads me to the question, if 100k in the FB circuit doesnt tame to much the signal. May be a 56k is enaugh. But easy to find out. What you all have there in, Presence or not?

Best

Hans-Jörg

[hans-jörg]

I deleted an other stupid question.

Hans-Jörg

[Tillydog]

I'm not clear what you have done from the description...

If you have the NFB connected to the 10k 'tail' resistor with the other end (normally connected to the presence pot) to ground, then you will have about 2 x the NFB that the original has (and the PI balance will be worse than the original).

A smaller feedback resistor than 100k will give *more* NFB, not less.

If in doubt, build it like the schematic

Ken Fisher's instructions were to bias 6V6 and EL34 to the same *voltage* (-31 volts from memory, but it is in the documentation somewhere).

Andy

[hans-jörg]

Hi Andy,
K.F. recomended -33V for bias. I guess.

To the NFB: I dont wanted the Presence Pot because useless with PPIMV. So only NFB with 5k6 to ground from Tail (10k). and the hole in the plate/chassis I had from Presence is now Cut control with 4n7. Much better.
I only done one mistake before when I connected the NFB: I forgot to connect the PI tail to ground. Nevertheless it sounded very fine though a lot less volume
And thank you for cerrecting me with the FB amount. I allways thought the FB would be less with lower FB-Resistor

Best

Hans-Jörg

[Tillydog]

I think it says -30V in the 'Trainwreck Pages' document, but whatever, - the principle is the same

... NFB with 5k6 to ground from Tail (10k)

... I always thought the FB would be less with lower FB-Resistor

FWIW: The feedback resistor and that 5k6 tail resistor to ground form a potential divider. Whatever is at the junction of the 2 resistors is fed back as the NFB signal, so it's really the *ratio* of the two resistors that matters, rather than the value of either one.

Glad you're amp is working well!

Andy

[hans-jörg]

I understand.
But than it would be better to bolt the 10k tail direkt to ground - without an additional resistor - to keep it neutral (same as Mark done it I think).

Later on I could try to shape it down through this devider - when I have the gound sound

Best

Hans-Jörg

[M Fowler]

Hans,

What I did works very well but remember that thread where Richie was advising on alternate Presence wiring. I would not exclude his ideas, but what I did just grounding the tail resistor works well and the guys that play the amp at the local guitar store love it so I leave it up to other musician's ears not only mine. Some of my amps I do not like the feel or tone but then walks in this guy that can make the amp just sing. Give it a try it's an easy fix back to stock. I put in a 250kL pot for a cut control replacing the presence pot.

Mark

[Tillydog]

... it would be better to bolt the 10k tail direkt to ground - without an additional resistor - to keep it neutral

The total resistance in the PI tail (i.e. 470R + 10k + "5k" ) is important to the PI, so connecting the 10k direct to ground will be different to connecting 10k -> 5k6 -> ground.

If you don't want the NFB to start with, just leave out the connection between the feedback resistor and the 5k6 / 10k junction (but leave the 5k6 to ground).

Alternatively, if you like the sound with a lower total resistance in the PI tail, then you can use (say) a 560R resistor at the end of the tail with a 10k NFB resistor to get the same amount of NFB as you do with the 5k6 and 100k.

Note that your PPIMV will affect the NFB response, also.

Andy