Author Topic: Fender T (issues)  (Read 3084 times)

0 Members and 1 Guest are viewing this topic.

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Fender T (issues)
« on: April 02, 2008, 04:06:05 pm »
OK, first off, I just snagged a well-used Fender Model T from a guy on the HC forums. He supposedly forgot to throw the footswitch in the box, blah, blah. I doubt it'll ever show up in the mail at this point. I put up a WTB for one, but it doesn't seem likely to happen. In any case, I'm sure one of the lame-o fender ones would work, but I'd really just prefer to throw one together myself. Would anyone happen to know what type of switches they used i.e. latching or momentary? And would it be the tip, ring or sleeve to control channel switching and fx loop switching respectively?

Second issue I'm having is with the fx loop and it's dull dull tone. Even with just a 12" patch straight from the send>receive there is a very significant loss of high freq/balls/vibe, especially noticeable on the high gain channel. It's  a shame because I had such high hopes for running my phaser/delays/verb in a loop for once.

Up until now I've been a single channel, everything out front of preamp guy, which was kind of the point of picking up something like the fender T with it's ultra-modern options = something different. My point is that I'm a fx loop noob and I'm not sure how much of my disappointment is just from the nature of tube-buffered fx loop tone loss or if my well-used head has been too used or if I need to do some tube swapping.

Which leads me to my next issue:

Would anyone happen to know which V position is driving the fx loop? I figured it would have been V5 or V6 so I tried a few different 12AX7 and an ECC83S, but those V positions didn't seem to effect the loop tone at all.

I've been looking around for a schematic for this head, but haven't run across it yet. Anyone care to share?

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #1 on: April 02, 2008, 04:07:23 pm »
I threw a few nice pics of my score up on my website here:

http://www.kyletompkins.com/wp/?p=554

Offline loudthud

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1,167
Re: Fender T (issues)
« Reply #2 on: April 02, 2008, 07:41:17 pm »
V4B drives the send controls on it's plate and the signal that bypasses the effects loop circuitry on it's cathode. V5A is the send buffer after the send controls and V5B is the recovery amp. The signal is routed with relays. K3 selects which send/receive pots are active, K4 mutes the send and selects the bypass signal or the signal from V4B.

This amp has one of those mute circuits that momentarily mutes the signal when relays are engaged. JFETS shunt the signal to ground momentarily. There is one signal shunt at the input of V4B where a relay selects the active channel and another shunt between the side of K4 that selects the loop or the dry signal. I mention this because these circuits can be damaged in a way the could cause the problems you are having.

Try connecting the cable from the send jack to the Ext Amp in, that will bypass V5B and the mute circuit described above. You may have to insert a plug in the Return jack to enable the send jack via K4.



Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #3 on: April 02, 2008, 08:34:27 pm »
V4B drives the send controls on it's plate and the signal that bypasses the effects loop circuitry on it's cathode. V5A is the send buffer after the send controls and V5B is the recovery amp. The signal is routed with relays. K3 selects which send/receive pots are active, K4 mutes the send and selects the bypass signal or the signal from V4B.

This amp has one of those mute circuits that momentarily mutes the signal when relays are engaged. JFETS shunt the signal to ground momentarily. There is one signal shunt at the input of V4B where a relay selects the active channel and another shunt between the side of K4 that selects the loop or the dry signal. I mention this because these circuits can be damaged in a way the could cause the problems you are having.

Try connecting the cable from the send jack to the Ext Amp in, that will bypass V5B and the mute circuit described above. You may have to insert a plug in the Return jack to enable the send jack via K4.

Awesome response! I'll give the bypass described above a shot and try swapping V4 - you know, all the layman stuff. The possibility of relay damage I'll leave for my tech to sort out.

Me and high voltage are like oil and water. My wife won't let me progress beyond building pedals, etc, unless it's strictly theoretical progression. I'm currently prototyping a boost/fuzz pedal and somehow I manage to walk away from my workbench with cuts, scratches and stab wounds galore, so getting hurt just seems to be my style unfortunately.

Do you have a schem for it or was this just from memory?

Offline loudthud

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1,167
Re: Fender T (issues)
« Reply #4 on: April 02, 2008, 08:47:02 pm »
I have it on CD but I think you can download it here:

http://www.mrgearhead.com/

Click the Sunn logo, then click Manuals.

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #5 on: April 02, 2008, 09:47:57 pm »
I have it on CD but I think you can download it here:

http://www.mrgearhead.com/

Click the Sunn logo, then click Manuals.

Is that your polite way of telling me to just shell out the $5 for the schematic?  :-D

I tried running the cable from fx send to ext. amp in (a plug in the return was necessary to close the circuit). It seemed to have resolved some of the tone loss /noise issues, I think, but it's hard to A/B, because I no longer have fx loop switching. I get no signal at all without the fx loop turned on, so if I wanted to run it like this I would have an always on loop. Does this shed any new light?

Offline loudthud

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1,167
Re: Fender T (issues)
« Reply #6 on: April 02, 2008, 10:07:20 pm »

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #7 on: April 02, 2008, 10:56:12 pm »
Just download the pdf file.

http://www.sunnamps.com/pdfs/manuals/guitar/Sunn_Model_T.pdf


Oh, I have the manual, but it doesn't answer any of my questions re: what type of switches/wiring I need to build a footswitch and my questions regarding V positions for the fx loop. You helped me out greatly with the latter.

I was hoping to be able to find the schematic.

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #8 on: April 03, 2008, 10:42:56 am »

Bump for footswitch info.

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #9 on: April 03, 2008, 04:33:07 pm »
AHA! Found it.

Is the footswitch the area in the top center that I circled? S1A & S2A would be SPDT correct? And why would they both be connected to the tip lug on the jack?



I'm assuming using dpdt instead would be the simplest way to add LEDs?

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #10 on: April 04, 2008, 09:42:55 am »
Something to do with pos and neg AC voltages activating the switching?

Offline kismet78

  • Jr. Member
  • **
  • Posts: 11
Re: Fender T (issues)
« Reply #11 on: April 07, 2008, 01:14:07 am »
Alright, finished with the DIY footswitch. Thanks for everybody's help. More photos and a build/finish report on my site: http://www.kyletompkins.com/wp/?p=573