6L6 to KT77

[azrael]

Hey, I have a Peavey VTM60. What would I need to do to make it take KT77s.
I have already modded it to have a bias pot.

[pdf64]

You would need to confirm that the power transformer is rated to manage the extra load on the heater winding that KT77 present, compared to 6L6.

[azrael]

Hm...how do I do that?
It's just the stock Peavey transformer.

I think some people have converted to EL34, if that helps.

[Doctordog]

Hello,

The KT77 and EL34 both have heater current draw of 1.5A, the 6L6 has heater current draw of .9A. So a pair of them will increase your current heater draw to 1.2A over what the transformer was designed for. This is not to say that some haven't tried it successfully, just that you are taking a risk of burning out your heater winding on your PT. Most companies do not overspec their transformers greatly because that would cut into their profit.

JB

[azrael]

So I would have to replace my PT if I wanted to do it?
Interesting.

I don't think any of the people who converted their VTM to EL34 changed theirs, I hope they don't experience any problems after long term usage..

[Doctordog]

Well, there is a way to be sure. If peavey would tell you the specs for the wire used in the heater winding of the power transformer. You could cross check the gauge of the wire with it's power handling capability. It's also possible that the PT is used in more than one model of amp and it has a heavier winding to handle another model that perhaps uses EL34's?
From what I have read, Peavey is very free with their information, maybe they will let you know?

Hope this helps.

JB

[azrael]

Cool, I'll email them and see what they say.

Do I have to change anything else like the screen grid resistors, or any of the stuff connected to the tube socket?

[Groover]

You could also add a dedicated filament supply transformer with more capacity.

[Doctordog]

Good point Groover, I had forgotten about that option.
As far as other changes needed, to my knowledge, there are none that I know of. Others may be of more help in this area than I. I do know that pins 1 and 8 need to be connected just with a jumper, they may already be and may not if the amp wasn't designed to take anything but a 6L6.

JB

[HTH]

There is always the possibility that the PT used is the same one used in some of their other amps that use EL34s - it's cheaper to use the same PT across a few amp ranges than have separate PT specs for each amp.

That said, I'd try the EL34s in there and measure the heater voltage before and after. If it drops down too much, I'd say the PT isn't liking the extra current draw (also monitor how hot the PT gets before/after swapping from 6L6s to KT77s).

[DrRick]

To be safe, you could add a small 6.3 volt 1.5 amp heater transformer to power one of the output tubes, leaving the other on the original heater winding.

[tedmich]

a 700-750 ohm 5w screen grid resistor probably wouldn't hurt for this mod (thats what the JSX runs for 6L6/EL34's)

two 3w 1.5k in parallel work well to handle the heat better
(Mouser: Vishay PR03000201501JAC00 0.49 each)

An Antek AN-0107 would likely keep your heaters happy for only $10 in a 2.4"x1" package that fits into chassis corners pretty well

[jrfrond]

I have a more pragmatic approach to this issue.

Do the conversion, and let the unit cook for a few hours. If the transformer doesn't overheat, then you are good to go.

I roughly define overheating as not being able to hold you hand on the transformer for at least 30sec. before feeling uncomfortable.

Not terribly accurate, but practically, it works.

If this amp had four power tubes, I'd be concerned. With two, it will probably handle the extra current draw just fine.

I'm not convinced that the KT77 needs higher-value screen resistors. Like the 6L6, it's a beam-tetrode, not a power pentode like the EL34. It's actually like a 6CA7. However, if you do want to give EL34's a shot, I would up the screen resistors to 1K/5W.

[tedmich]

I have a more pragmatic approach to this issue.

Do the conversion, and let the unit cook for a few hours. If the transformer doesn't overheat, then you are good to go.

I roughly define overheating as not being able to hold you hand on the transformer for at least 30sec. before feeling uncomfortable.

Pragmatic AND (potentially) cheaper...I love it!

I am going to slap some KT77s into my PV Ultra 60 this week!

are other appendages more sensitive to tranny heat?

(kidding)

[Guitarist]

How are the KT77s treating you guys with the VTMs? I'm about to up the screen resistor to 1.5k or 2K and also add some shielded wire from the input jacks.
Also might, go Friedman on this thing and swap some 300k plate resistors on V1.