Fender 'Twin Amp' c2002

[Tom Roberts]

I bought the Fender 'Twin Amp' from a shop and it was nearly new. Not long after, it began blowing fuses. This fuse lies in an in-line fuse holder on one of the wires coming from the transformer and connects via a spade connector to the circuit board; this wire is Green and the place where it is supposed to connect to the board is clearly marked VERD/GRN. However, after it has comes through the PCB, it is connected to a RED wire by means of a male-female spade connectors and this red wire carries the in-line fuse. Late last year, the fuse blew again but this time, the in-line fuse holder melted and had to be replaced. It is rated at 10amps.

Having read a few amp repair and modifying books by some well known authors, I decided that the problem that I was having with my 'Twin Amp' was being caused by a/some problematic axial electrolytic capacitors so, I change all 9 - 5 rated @22/500 and 4 rated @47/350.

The problem? After being on for approximately 1.5 hours, the power transformer becomes so hot that you cannot put your hand on it or within an inch of it.
All valves, both 7 pre and 6 output have been changed twice in the last few weeks, all to no avail and the bias has been twice set correctly also.
Anyone - any suggestions?
Regards Tom

[MWJB]

The green wire comes from your power transformer? The only green wires that come from the PT are the heater wires (no fuses shown on current schem), this might be the only circuit that I would expect to finda 10A fuse in (it's not going to protect any other circuit in the amp at that size).

This fuse sounds like it may not be original? Could be a band-aid attempt at a fix.

I'm not convinced that the filter caps were at fault, double check that you fitted the new caps at the correct polarity. No possible shorts at connections.

You say bias is "correctly set? Well if you set it, then you should know what it is, please confirm actual measured plate current. Should be around 30mA/mV per tube, or 60mA/mV per pair/side.

The PT is getting hot because there is a large demand on it, maybe too much plate current, maybe a short somewhere (power tube, backwards filter cap).

Pull out all the power tubes, power up, any fuses blow? If no, power down, install one power tube, power up any problem? Keep going until you find a tube that makes a fuse blow (bin that tube)...if a particular socket, with any tube, makes the fuse blow then you may have a socket/PCB issue. Obviously, being a 4 output tube amp a new, matched quad of power tubes would be a good idea, you may be able to use any known good tubes from the previous set as spares. You don't need to run all 4 power tubes for troubleshooting purposes.

"All valves, both 7 pre and 6 output have been changed twice in the last few weeks" Surely only 4 output valves?

[Prairie Dawg]

The green wire comes from your power transformer? The only green wires that come from the PT are the heater wires (no fuses shown on current schem), this might be the only circuit that I would expect to finda 10A fuse in (it's not going to protect any other circuit in the amp at that size).

This fuse sounds like it may not be original? Could be a band-aid attempt at a fix.

I'm not convinced that the filter caps were at fault, double check that you fitted the new caps at the correct polarity. No possible shorts at connections.

You say bias is "correctly set? Well if you set it, then you should know what it is, please confirm actual measured plate current. Should be around 30mA/mV per tube, or 60mA/mV per pair/side.

The PT is getting hot because there is a large demand on it, maybe too much plate current, maybe a short somewhere (power tube, backwards filter cap).

Pull out all the power tubes, power up, any fuses blow? If no, power down, install one power tube, power up any problem? Keep going until you find a tube that makes a fuse blow (bin that tube)...if a particular socket, with any tube, makes the fuse blow then you may have a socket/PCB issue. Obviously, being a 4 output tube amp a new, matched quad of power tubes would be a good idea, you may be able to use any known good tubes from the previous set as spares. You don't need to run all 4 power tubes for troubleshooting purposes.

"All valves, both 7 pre and 6 output have been changed twice in the last few weeks" Surely only 4 output valves?

+1 on the fuse. That shouldn't be more than about 4 a. As to temps, go to Harbor Freight or some such cheap tool place and get yourself a digital infrared thermometer, the keychain typme made by Centech-it will cost you about ten bucks. Never be in doubt.

If it is as hot as you say, the amp will die in fairly short order. Expect smoke and bad smells a plenty.

[Jazz P Bass]

Fender The Twin

The two power transformer wires are indeed the Vac for the tube heater filaments.
Pull all the tubes and, with the power on, measure the Vac across the two green wires.
You should read 6.3 Vac.
With the power off, measure the resistance to ground of each green wire, one at a time.
Should read 47 ohms.
One at a time install the preamp tubes while monitoring the Vac across the two green wires.
If any tube makes the Vac drop, that tube is bad.
It may very well be a shorted winding in the power transformer.

[trem]

Is that the right schematic for the amp he has?

[Tom Roberts]

Hello,
Thanks for getting back to me so soon. You're right: there are only 4 tubes!!
All 3 sets of 6L6s that I own are SED winged Cs and all were interchanged in each trial. That is, I didn't just use the 4 that came in the group that I bought. I've also done the remove and replace each tube and fired it up again and waited. Also, I tried each one in my JCM 900 50W and all were fine. All caps were corrctly fitted (double checked) as the positive is clearly marked on both caps and on the PCB. The quad of 22/500s (2 on top of 2) were fitted yesterday and the 4 x 47/500 (could not get 350 vdc as originals) and 1 other 22/500 were fitted this morning. The originals were Illinois' and the new ones are German F+T. Rechecked - polarity is correct on all.
The bias was set by an electrical engineer using information supplied by me at 60mv. I got the information about bias settings from 2 sites on the web, one in the UK. Apparently, there are a couple of differences in chassis mountings and mine is the one with 8 mounting screws on the top and 2 on the side and this means that the bias settings is different to one with less mounting screws (?)
As to the wire coming from the transformer, there are 3 REDS, 2 BROWNS and
GREENS. These are without doubt the original wires from the PT and the other RED wire, the one with the in-line fuse holder on it, is an addition and is connected to one of the original GREEN wires by means of spade connections. By this I mean that, one of the GREEN wires that leaves the PT beneath the PCB then becomes a RED one with a 10a fuse by the time it connects to the board.
Tom



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[Tom Roberts]

I'm no electrical engineer but _ I tend to agree with you, something that I've thought about for months - that there may be a short inside the PT! The schematic that you sent does NOT correspond to the one on the cab.
V1 - 12ax7a V3 - 12ax7Wa V5 - 12ax7Wa V7 - 12AT7 V9-12 4x 6L6
V2 - 12ax7a V4 - 12ax7Wa V6 - 12ax7Wa V8 - 12ax7Wa

Regards Tom

[MWJB]

"Apparently, there are a couple of differences in chassis mountings and mine is the one with 8 mounting screws on the top and 2 on the side and this means that the bias settings is different to one with less mounting screws (?)" curious, I've never thought to set bias based on the number of mounting screws an amp has...it's a novel approach, I'll give you that :-)

Aha! Just spotted a T5A heater fuse in the UK/Aus Export schem. so it looks original. Get that 10a out of there.

Ok, so where are we at? The amp now functions OK, with 4x known good power tubes, without blowing fuses? The main (primary 2A if you're in EU?) fuse never blows?

Power up with no tubes in again, measure heater voltage, should be >6.3VAC (more like over 6.6VAC), power down & install a power tube, power up, all OK? As you fit more tubes the voltage will drop a little, you're looking for tube that causes it to drop a lot/blows the fuse.