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6V6's in a HIWATT?

7/29/2004 4:24 AM
chris 6V6's in a HIWATT?
got a 50w HIWATT but it's really friggin loud...  
I know some people use 6V6's instead of EL34's in old marshalls, has anyone tried them in a Hiwatt yet? any reason why they wouldn't work?  
I guess if the plate voltage is in the 6V6 range there's no reason why not?  
7/29/2004 10:29 AM
Steve L

I know this forum is all about mods, repairs, etc, but if that HiWatt is from the 70's, PLEASE don't mess with perfection. Just get a power brake or something to lower the volume.
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7/29/2004 2:32 PM
I can't see how changing power tubes to a different type is going to adversely affect the amp..except for maybe rebiasing?  
I know that in the early '70's marshalls, it's pretty straightforward to swap the EL34's for 6V6's  
I agree with you though, there's no reason to go in & start rebuilding the amp, it does sound great!  
7/29/2004 3:37 PM
Rob Mercure

Hey Chris,  
What model HiWatt do you have - and do you have a schematic? Have you measured your plate and screen grid voltages - might as well provide us with g1 bias also.  
Swapping as 6V6 for a 6L6 is often successful but the EL34 - as originally designed - had maximum plates voltages of around 750-800 VDC (as opposed to around 350 for the 6V6 and 450 for the 6L6GC [these are general notes - specific max Ep for the tubes changed over time with method changes]). So, plugging a 6V6 into a 6L6 socket with 400V on it is considerably different from plugging it into an EL34 socket with 700V on it. And HiWatt was known for high plate voltages. (This is why specific measurements from your particular amp are necessary. You may be one of those folks who has power line voltages around 125 VAC which would give you super high B+ voltages, for example.)  
If nothing else you're going to have to install a variable bias supply for accomodate different tubes - generally a good idea no matter what you decide - and perhaps even increas the bias with a voltage doubler, not uncommon with HiWatts. Probablyt also need to increase the screen grid resistor volue and current rating - and perhaps limit the g-to-g signal swing on the output tubes.  
Assuming the circuit can be made friendly for 6V6 you're talking about few hours with a soldering iron. Luckily what I've described will improve the amps reliability and give you better tube replacement options.  
7/29/2004 5:02 PM

According to target=_blank>the power supply schematic there's only supposed to be 460V in a 50W 4-input Hiwatt, which, of course, will be higher if your wall puts out more than 117VAC....  
If you need to cool off the bias, you can hook a 1M resistor in parallel with the 110k resistor in the bias supply (it's similar to a Fender Princeton's, with a resistor & diode off the B+ winding) to cool things off a bit. You'll be fine then if you use good 6V6es. I recommend JJ's nice 6V6es--they're really beefy, and, IMHO, have great tone. Oh, and no, I don't work for JJ.... :D  
Also, you'll want to run a 16 ohm speaker on the 8 ohm tap. The 4 ohm tap, IIRC, on some models is really the same tap as the 8 ohm tap, so it won't match up if you run an 8 ohm on the 4 ohm tap if you've got one of those. Apparantly there are two different 50W OTs common to these amps....  
Here's target=_blank>the output section and target=_blank>one of the 4-input preamps. All schematics can be found on target=_blank>Mark Huss' web page  
C ya,  
7/29/2004 6:06 PM
hey, thx guys!!  
much appreciated  
I just got the amp yesterday, so I'll give it some time. it does sound amazing as-is...this just sounded like an inexpensive & ( potentially)harmless idea to try w/ the volume  
I'll look into the details  
7/29/2004 6:22 PM
Swami Rabinowicz

If it were mine and I were to try this I'd go about it a little differently.  
I'd up the screen resistors some...probably to 1.5k/5W or even 2.2k.  
And I'd cathode bias it, to relieve the tubes of some of the high plate voltage.  
This would be pretty simle and should be easily reversible.  
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