reissue T... 6L6 to 6550... etc.

[JoeArthur]

Quite a few posts are around this subject, so I thought I would toss in my two cents here instead of each one of those posts.

First of all, there is no such thing as a "reissue T".  The Fender Model T is a completely different amp from the Sunn Model Ts.  They only share the same name - nothing else!

Can you convert a 6L6 Fender Sunn T to use 6550/KT88 tubes.  Yes, but it is not an easy or inexpensive process.  

The heater current needed by a 6550 is 1.6 amps each.  The heater current needed by a 6L6 is about 0.9 amp each.  So the heater current needed by a 6550 is not quite but pretty darn close to twice that needed by a 6L6.  Four 6550s will be trying to draw 2.8 amps more from the 6.3 volt heater supply than 4 6L6s.

Do I believe that Fender built in almost 3 amps of additional capacity in their power transformer?  NOT with their cost cutting ways...

If you have bucks to replace the power transformer when none of your tubes light up any more... go for it.

So you or your tech will first have to supply the required heater current.  This could be done with an extra 6.3 volt heater only transformer...

The output transformer should be able to handle the 6550s.  It probably won't be as efficient as it could be.

But now that you've gone this far... you might still be able to get 100 watts out of the amp.  To increase that, then a new (and different) power transformer will be needed to increase the amperage of the B+ supply to the output transformer... and since you've done that, you might as well replace the output transformer!!

Oh... we haven't talked about trying to bias the new 6550 output tubes... well, maybe we can assume that was taken care of with the new poer transformer.

Do you see where I'm going with this?  Why would you want to do any of this in the first place?  It would be much cheaper and certainly easier to buy a real Sunn Model T.

[edward/harmal]

hi, thanks for the thread, very informative!!
the main reason I am considering modding my fender is because it is in the shop.
it isnt turning on at all right now for some reason, so i figure if someone needs to work on it i  might as well try and go for more tone... kt88s would probably sound awesome

one of the selling points of the fender t is that it is 2 channels footswitchable, and the gain is not your average mesa/marshall high gain saturation, the fender sunn has a pretty raw, thick distortion channel with sweet mids and a round fat sound,
 through the wrong cab it can be muddy and undefined. i'm using it in an ampeg 4x12 cab with celestions 2 vintage 30s and 2 anniversary 30 watts and it sparkles

i appreciate someone clearing the retube situation up though, thanks joe

im not sure if i'll mod my amp yet, but it would be nice to hot rod the reissue and get it sounding better.
 the prospect of a slightly modernized, footswitchable retake of vintage Sunn tone sounds too sweet to pass up.

i've never played a vintage T but i am sure it would blow my mind. without finding one locally the only way i can get a real Sunn is through ebay, and im a little uncomfortable about buying a vintage tone monster online

i do have just a couple questions i would appreciate help with though if anyone has this information

  how much do you think a 6.3 volt heater transformer costs? any particular kind of 6.3 volt heater needed for this amp?
 what type of power and output transformers are needed for the fender model t and how much do you think replacements would cost?  are these things hard to come by?

can you help generate more clean headroom or alter the tone by upgrading 6.3volt heater/power/output transformers or are they pretty inconsequential?

would the gain channel be affected by the replacement tubes? im thinking it would light up and sound awesome. eurotubes recommends kt88s as a replacement for the 6550s..  are the kt88s really the best choice for tone? would i need to buy new sockets for the tubes to plug into or would they just sit in the 6L6 slots?  
 

a vintage sunn would be cool but if all of this stuff + tubes + labor is not too expensive, im just fiending for sweet tone and the footswitch would mean one less bullshit boutique distortion pedal on my board

ok any advice would be appreciated, thanks guys , it would be nice if this could work out, right now im waiting to see exactly why my Fender wont turn on, depending upon whats wrong with it i will either get it running with KT88s or i will get it so it turns on and ebay it for a real sunn model t or a hiwatt (so sweet)
thanks again

[JoeArthur]

Expensive is relative.  You could easily spend $300 on replacement transformers alone.  If you replace the power transformer, hopefully your tech would get one with sufficient current capability so an additional heater-only transformer isn't needed.

I believe this is a PCB built amp, so modifications aren't going to be easy - that translates into more time and therefore more money paid to the tech.  This could easily cost you $600 or more.

Before you do that, you might want to try a little test drive to see if you are going to like it.  

Remove all four 6L6s and replace them with ONLY TWO KT88s (there is sufficient heater capacity for two and DON'T try to do the JJ 6L6/KT88 5150 mod either - you might fry the xfmr) and adjust the bias (it will need about 10-15 volts more negative than the stock setting for 6L6s on the grid, so ask your tech to check it out, and he should know how to modify it for move bias voltage if necessary).

Then take it home and live with it for at least a month.