That looks really cool, CJ.
Just had the opportunity to test the amplifier yesterday. Clean channel works, boost channel - no joy!
Not sure what's wrong. I checked every passive components on board prior assembling it back, and they are all ok.
The tube was glowing dimly.
View attachment 123220
Stupidly I didn't bring my multimeter with me, so I wasn't able to take any voltage measurements.
The board is now back on my bench, minus the tube and the main transformer. Couldn't carry everything with me as I had to take a ride on a crowded subway train.
Anyway, I saw two videos on YT in regards to these 68 ohm resistors. One by
TampaTec where he said they are connected in parallel, measured 34 ohm then again 68 ohm after he cut one leg, but in another video by
denbydish where he restored the burnt out tracks, I saw they are not connected in parallel, the same as the schematic and the same as the board that I'm working on. Unless there are two different board revisions, which one is correct????
Aside from that, what should I be looking at in order to repair the boost channel? I'm not familiar at all with tube circuit, but I suspect the tube is not glowing bright because it doesn't have the high DC voltage supply (AC from autotransfomer T5899 rectified by D3)?
I have a 230VAC to 2x18VAC main transformer, can I use this temporarily to check if the autotransformer is working or not?
Oh, do I need to start a new thread?
I'm in a similar position with gear repairs after getting back into electronics and finally getting my little lab set back up. Life certainly gets in the way at times, and my priorities were certainly not in this area, but I'm really enjoying it now and will finally get around to finishing off many of the projects from here.
I've succesfully repaired a lot of amps recently, and although all transistor etc, I've got a couple of valve amps to look at, but I've been putting those off till I was a bit more up to speed so to speak and in the meantime, have been learning more about valves.
Anyway, I'm waffling, so I'll do my best to give you a hand and happy to be corrected by the more experieced members on here.
I've taken a look at the schematic and heres a few things to test.
Given that the amp is basically working, that would tend to indicate the PSU is functioning on the low voltage side(power to opamps) and the rails to the power amp section are ok and hence the transformer secondaries. Check to see if the led for the boost channel lights when you switch channels. If not, try inserting a jack into the channel switch skt and short it out (rule out the front panel switch). If there is still an issue, then check the voltage rails on the opamps for the boost circuit. I pulled up a data sheet for IC1/4 as I wasnt familiar with an M5201 opamp and it also basically showed me how the boost circuit comes into operation. These use a control voltage (pin 1) to basically switch them on/off (hence boost circuit in/out), so check pin 1 when switching channels. This should rule out the channel switching circuitry.
Quick test for the valve would be to swap it with a known good one (sorry for stating the obvious), but if you havent got one to hand then, a bit more testing.
The ECC83 (12ax7) has two heaters that can be wired to work off either 6.3V (in parallel) or 12.6V (in series). Current draw is about 300mA using 6.3V, or 150mA using 12.6V. Looking at the schematic, the heaters are wired in series and the circuit uses the two 68R resistors (R62/63) to drop some voltage. Given that, and using ohms law, the voltage dropped across each resistor will be 10.2V (0.15x68) or thereabouts, which would suggest a voltage of about 33V (10.2 + 10.2 + 12.6) across the secondaries of the transformer. This also shows a power dissapation of approx 1.5W for each resistor, hence doubling up to 3W rating. The transformer is supplying the rest of low voltage rails so that should be ok, but check either side of R62/63. Pull the valve and check the resistor values (looks like youve replaced them so should be ok) and no, these wont affect the sound of the amp. I recently repaired a power amp that had gone into protect, which was caused by dropper resistors for the LV supply off the HV supply going open circuit on the -ve rail, but you'd never know by looking at them.
The HV supply can be checked by taking a DC reading on the LHS of R64 which will rule out the 'step up' secondary and D3. From there you can follow the circuit and check the 2 plate voltages at R53/54. Be careful!!!
If all of the above seem ok, then it could be time to try a new valve, or if you have a scope, then trace the circuit through for a signal on the output of the opamps in the boost circuit which renters the signal chain just before the FX send jack.
As always with amps, good to check all manner of things from bad connections on jack sockets (FX send and return notorious for faults) and dry joints etc. A chopstick and tapping the circuit board will often show these up.
Hope thats of some help.
Here's the schematic for anyone interested and data sheet for the M5201.
PS, re-read your posts and see you've got the step up up TX, for the HV supply out the board. You really should get that back in to test as its not indicated on the schematic what the intended plate voltage is. An 18V TX wired backwards could be ok(probably in range for an ECC83) but obviously a risk to the rest of the circuit. I wouldn't want to be experimenting with that on a customers amp.
