Tube Amp Archive

I was a little perplexed that despite having the same part number, the power transformer in AA769 Twin Reverbs do put out significantly less voltage than those in the AB763. As a result, short of swapping out the power transformer, it's not going to sound like a AB763. With that in mind, would this ...

I've been experimenting with this thing all day and it still doesn't sound right to my ears. I have the bias supply converted from the mixed cathode / fixed / balance circuit to that found in the AA864. I also converted the phase inverter to the AA864 specs. The bias mod is a no-brainer. From what I...

Thanks for the replies. I've since found one of the coupling caps in the bass channel is leaking DC pretty badly. I also read that a leaking coupling cap can cause excessive gain in the following gain stage. I'll change it and report back. Another symptom is that the deep switch makes an unnerving "...

I just read an article and think I have an answer to one of my questions. In changing the bias supply circuitry, I changed the phase inverter coupling caps from the stock 0.047mfd to the blackface 0.1mfd value. I think that this change should be only be be done in tandem with rewiring the remainder ...

I was recently restoring a Twin Reverb and a few people here helped me troubleshoot a few issues (Thanks!). The amp is now restored and sounds absolutely beautiful. Here are a few photos if anyone is interested: https://plus.google.com/photos/113594007843788366813/albums/6001129928097595905 I'm now ...

Since you likely have too much negative bias anyway, try a 1K to 3K3 dropping resistor and see if comes down to a safe range. I don't have any new resistors of the appropriate value so I dug out the one I took out of there in the first place. It's supposed to be a 3.3k, but had drifted to about 3.5...

Firestorm wrote: Raw bias is DC you have at the negative end of the bias cap. You don't want it to exceed your cap's voltage rating.

I just checked it and with the 470R on the bias board, I'm getting -66V through that cap. I'm definitely going to need to change something! That's over 30% over its rated capacity.

Something just occurred to me: When I started this, the bypass cap connected to pin 3 of the vibrato tube was not on the board per the layout; it was wired point to point from pin 3 to the vibrato pedal plug. When I replaced the cap, I put it on the board per the layout, but now that I think about i...

martin manning wrote:If a transformer has shorted turns it will overheat even with no load connected. Too hot is too hot to touch for more than 5-10 seconds; a faint hum and warm to the touch is normal.

I'll check and report back.

I'd just adjust the shunt resistor unless the raw bias is over 50V, since you only have 50/50 as a bias cap. I don't have all of the lingo down yet. The shunt resistor is the one on the back of the bias adjustment pot, correct? By "raw bias" you mean the AC voltage coming from the PT? Mine was 52.6...

I'm wondering about the PT. You are right voltages should be high rather than low. Does it seem to run hot? What happens if you just leave it on with no tubes installed? I don't really have a reference for what is "hot" for a PT. When I was playing around with it a lower volume, I think the PT was ...

Some the late 60s Fenders ran lower voltages. My AA769 schematic shows 405VDC on the plates. With AB763 values in the bias supply, the bias voltage may always be too negative. Good catch! I never thought to check; I just assumed they changed the circuit without changing the power transformer. Compa...

I got some work done last night and this morning: Cleaned the tube sockets and jacks with DeOxit. Cleaned up the lead dress around the phase inverter and tremolo driver tubes. I used the AB763 layout as a reference. I listened to that oscillation more closely so I could hopefully describe it in more...

Yes, by using the somewhat-advanced OT primary shunt technique, where you put the meter probes across each side of the OT, from plate lead to CT lead, shunting virtually all of the current through the meter. It's nice to have a "hold" feature for this since you don't want to be looking away from wh...

Thank you all for your replies. As promised, here are some photos of the chassis: https://plus.google.com/photos/113594007843788366813/albums/6001129928097595905 I apologize for the quality; they were taken with my Nexus 4, which has a rather poor camera. I suspect the lead dress near the phase inve...