6v6s & 6L6s in same amp - 5E3 project update

[stephenhartley]

Firstly, I'd like to give an update on the 5E3 based project that I built & once again thank everyone on here who kindly gave helpful advice. I've now used the amp at several rehearsals including one in a rehearsal studio with (loud) full drumkit and at 2 small venues (100 capacity) and it's superb - the sound carries beautifully and there's plenty of body. At the first gig, I was approached by an electronics engineer who wanted to know everything about it. It holds its own against my Vox AC30 and 64 Tremolux. At low volume it has great clarity and definition. Its only failing is that it's no good for bass guitar - the sound is fuzzy and flabby. I suspect that this is because the top 2 speakers are probably not proper fullrange - they're a pair of Alnico 6" Wharfedales - I'm aiming for a 4x8 amp, but I had these 2 speakers so wanted to give them a try.



Secondly, I want to build a 5E3 (or 5E5) amp that will take either a pair of 6V6s or a pair of 6L6s - from the research I've done so far, the PTs and OTs are crucial - is it possible to use PTs and OTs that have enough 'slack' for both or is this just naive? - There are a few threads on here, but none seem to answer the question definitively

[Chuck H]

It's entirely possible. Just build the amp for 6l6's. Some people think that over reated iron is a detriment to tone. I don't. But everyone is entitled to their opinion.

Three things that need to be considered and designed into the amp are:

1) Plate voltage should be reasonable for both tubes. For an amp that will run 6l6's or 6v6's that basically means you don't want too much voltage for the 6v6's.

2) The OT primary impedance is a touchy subject for some. I'm not in that camp. I would probably use a 6.6k primary impedance and call it good. Others would probably use an 8k or 4k primary and use the secondary impedance taps to reflect 8k primary on the 6v6's and 4k on the 6l6's. Still others might use base the primary impedance on plate voltage, tube dissapation and bias condition. Way overkill for a guitar amp IMHO.

3) The bias condition for either tube type must be accomodated.

There's really nothing more to it than building a 6l6 amp that doesn't have to high Vp for 6v6's, use a primary impedance that's good for both tube types OR have the primary impedance selectable for both tube types and be sure both tube types can be biased properly.

[stephenhartley]

Thanks for your reply Chuck

As coincidence would have it there's an OT on (UK) e-bay at the moment that has both 6.6 and 8K primary impedance, so would allow switchability between the 2.

I presume that if I choose a PT suitable for 2 x 6L6s, then compensate for 2 X 6v6s by not biasing them too hot, then have two switchable cathode resistors, that would do it.

I know from Weber's book that 6V6s like 30 to 40mA, but I don't know the equivalent for 6L6s

[greekie]

I have actually built an amp like the one you are talking about doing.

I made a couple of things switchable;

1) A general 6V6 / 6L6-switch. It switched the following elements:
1.1) Cathode-resistors on the cathode-bias mode.
1.2) Bias-supply voltage in the fixed-bias mode.
1.3) HT-level. Basically used a MOSFET to drop the voltage from Fender'ish 6L6-levels to a more acceptable level for the 6V6-tubes.

2) Bypass-cap on the power tubes' cathodes. I went for a 3-way here too:
2.2) Full on
2.2) 330 ohms resistance between cap(s) and ground.
2.3) 100k ohms resistance between cap(s) and ground (basically cutting the effect of the cap without the huge pop through the speakers).

This, a long with a PPIV-MV and a good speaker made this amp a versatile gigging-beast. I hope you can put these ideas to good use. Good luck with the project!

Jake

[stephenhartley]

Thanks for that Jake

I don't understand the role of the resistors to ground from the cathode bypass cap and I don't know what a PPIV-MV is?

I'll look into the other points. I've found an OT with separate 6.6 and 8K primary impedance, so I'll include that in the 6V6/6L6 switch as well

[capnjuan]

Thanks for that Jake

I don't understand the role of the resistors to ground from the cathode bypass cap and I don't know what a PPIV-MV is?

I'll look into the other points. I've found an OT with separate 6.6 and 8K primary impedance, so I'll include that in the 6V6/6L6 switch as well

Hi stephen: PPIV-MV is a post-phase inverter master volume control.

In a cathode-biased amp, the amp's bias is set by inserting a resistor - in parallel with the cathode cap - between the cathodes and ground. That pushes up a small voltage on the cathodes assuring that the cathode will always be more positive than the grid ... so current will flow from the plate to the cathode.

In a fixed-bias amp, a small negative voltage is injected onto the grid causing the grid to be more negative than the cathode ... or that the cathode is more positive than the grid ... either way ... cathode-biased or fixed-bias ... the cathode has to be more positive than the grid. Good luck with your project!

[capnjuan]

Forgot to mention: 6L6 heaters draw almost twice the current of 6V6 heaters; need to make sure the heater winding in the PT is stout enough to support the added draw. cj

[stephenhartley]

Thanks for the help. I'll keep posting on progress (don't wait up, my first project took me 2 years)