Adding another tube to a Crate GXT100

[screamersusa]

I'm toying with the idea of adding a unity gain tube input buffer
built into the back of my amp. Not sure what I'll do with the other half.
I'm concerned the power supply may not be able to handle another tube due
to postings regarding the power transformer failing.

I notice a big difference using a prebuilt unity gain 12ax7 buffer at +-15v before the opamp
input section on the crate. I becomes more organic and has more of what I was looking for
in some of my Boogie mod attempts. I'm trying to keep as much in the amp as possible
and will simply use in/out jacks on the back to use the buffer, but it will live in the amp.

If someone has a schematic for a nice unity gain tube buffer at those voltages, It would be appreciated.
I might use the other half as either an overdrive or pre buffer for a MK2 eq (also mounted in the back.)

[soundguruman]

have you lost your bloody mind?

[screamersusa]

have you lost your bloody mind?

Of Course, as always, but Not if it works.....
If I'm running an external tube buffer already, that has a +-15v and +6v wallwart into a hybrid amp,
then why not add an external in/out tube buffer powered off the amp at the right voltages but built into the chassis?
I'm still gonna run the amp input "as is" and just run the guitar through the tube (via 1/4 inch jacks).
Less boxes and plugs to carry. I have the tools to install another socket properly.
As it is shaping up now, the amp tweaks are resulting in only needing to carry, the amp, the footswitch, tuner, two effects boxes that all
live with the amp and run off one ac cord. The only thing in front of me on the floor is the footswitch instead of
3 dirt boxes, a chorus, tube buffer ,boogie mark2 pre eq and two delays.
That makes room for a TC megaphone, mic/pre/compressor, and a boss ve20 all for vocals.
I don't mind my drumkit being an insane beast, but I want my guitar rig as simple as possible.

If I could afford it, I'd have a Mesa Quad or Triaxis preamp, an Engl 530 or 620 preamp, two TC effects boxes an
octaswitch and a Crest in a rack. Since I dont have about $12,000 I guess I'm stuck with this amp.

I could simply bolt the existing buffer to the bottom of the chassis and power it off the amp, but I thought
it might be nicer to run a dedicated one at the right tube voltages. The existing one is a hifi or guitar line buffer kit
pre-made board that runs at low voltages 15-35v and I checked it on a spectrum analyzer for flatness.
It does add a "bell like" quality vs the input opamp alone. It's already a hybrid amp anyway

[Jazz P Bass]

So your main concern is the 12AX7 current draw of the heaters?

12AX7 triodes are rated at .3 A per heater.

I don't think anyone could tell you if the transformer can deal with the additional draw.

It's not like there is a published spec.

[screamersusa]

Yes Jazz P, I was concerned about the heater voltage. I know running the existing buffer off the unregulated
+-15v (we hope) will cause problems and I will be running 3 radio design labs modules off it already.
That .3a rating helps, thank you.

I have seen posts about power transformer issues hence the concern. My assumption at this time is that
people are blasting these amps or running them hot at 4 ohms till the mosfets blow and take out the PT.
There are only two power mosfet finals in there when I would have expected 4 for a 100w amp thats going to
be beat to death all the time by the end users.

[screamersusa]

Not Gonna Happen....... Idea good, space available, power supply wont take it.
Turns out they are using the heaters to regulate the +-15 after the +-38v for the power amp.
Not having regulation for the chips or a dedicated supply for them annoys me a little but it
is what it is. I'll bolt the existing tube buffer to the side of the case and redo my ac power distribution.
I went and tweaked the gain of the other half of the tube and stuck the Mark2 pre eq before it with an in/out
switch (unity at standard mk2 settings). The difference with and without the input tube buffer is quite noticeable affecting technique.

[J5&10's]

Just joined this site today ...so for your question it's been awhile.

I'm a harp player, so I have different needs for tone. Have owned the GXT-100 about 15 years, bought new. It was my regular gigging amp for quite some time.
Always was a challenge, being very gain-ey for harp. Plenty loud, though.
Eventually, I found the right 'tube soup' for me. I use a 12BH7 in the V2 slot, which tones it down considerably. The BH7 is a longer bottle, but not problem. It does have double the current draw of a typical 12AX7 of 0.6a vs. 0.3a , but I've encountered no problem. For V1, I use a 12DW7, a 12AY7, or a 5814A ( 12AU7 variant ) depending on the venue. All have heater current at 0.3a . This provides solid breakup and pleasant distortion without having to use a series of pedals or other FX chain. Reverb's great when I need it.

I rarely use the amp anymore, as I tired of lugging 48 lbs. Only concert venues. Have smaller amps that work fine: VHT Special-6, Blackheart Handsome Devil 15 piggyback and cab, Ampeg Rocket rebranded Noble Victor, two Gibson EH-125's.

One other thing: I did have to replace the transformer quite early, but it wasn't from subbing tubes. The holding bolts to the chassis loosened up, and the trans dropped while plugged in. Caused a short that blew it. You might want to check on those.
Had my tech replace with a slightly stronger transformer, then installed a mounting bracket to give added support.