Asteroid Inspired Build Troubleshooting

[DGNGuitars]

Long time lurker, first time poster. Lol. I’ve built several Fender and Marshall amps over the years and decided I needed a new project.

I put together an amp inspired by the Asteroid, except I wanted 2 channels, one clean, and reverb. The plan was to start with the stock layout and add the clean channel, switched by relays, and reverb from there. I figured get the core amp sounding good then add the extras.

So when I fired up the amp with power tubes after checking all voltages it was motor boating. I missed the ground from the 3 - 20uf filters caps. Whoops. Installed the ground and I have sound, actually sounds great, the only problem is there is just little of it. The amp has a fraction of the output it should. I’ve gone over the amp with a fine toothed comb about a dozen time. All voltages are within an acceptable range. Everything is wired properly. All the correct values of parts are used in the circuit.

I’m using Edcor Transformers as they were cost effective and this was to be just a bit of an experiment.

The PT is spec’d for 375-0-375 and I used a 5U4G rectifier to get the voltage into a range that the stock 365-0-365 with a GZ34 would be. I’m about 3-4 volts lower at idle than what the schematic I have says so that seems fine. And again all other voltages are well within Spec.

I’ve swapped the OT primary’s no difference, not even a squeal, then swapped them back. Swapped in 2 other sets of good el34’s, no change. Swapped PI tube as well.

So I figured maybe there was something wrong in my preamp section. I wired up my clean channel board, bypassing everything up to the PI and I get the same result, good sound, just not a lot of it. I swapped out the coupling caps in the phase inverter for another new pair. No difference. To me this points directly at an OT issue.

Is it possible that I got a bad OT out of the box? Could the lack of grounding on the 20uf caps on initial startup done some sort of damage to the OT? I’m not entirely sure how to effectively test the OT. The primaries read in the 300 ohm range and the secondaries 1.12 ohm on the 16 Tap and less for the other two.

Here are links to the transformers used:
https://www.edcorusa.com/cxpp50-66k
https://www.edcorusa.com/xpwr222

I know they are not exact spec but I figured they would get me in the ballpark. One last note, the OT has UL taps, am I safe to assume they are not used? I trimmed them short, shrink tubed the ends and stuffed them under the end bell.

I’m stumped and am hoping someone has some input. Also sorry for the long winded first post.

Thanks Dan

[xtian]

First step is to check voltages at all plates and cathodes. If they all check out, it's time to trace the signal. 1KHz sine wave in, see what it does after each stage.

[pompeiisneaks]

and if you don't have a scope you can put your DMM to Volts AC and see what kind of voltages you get at each stage. Every amp is different, but you'll get say an input of 100 - 400 mV sine in, and at the next stage several volts ac, then at the PI output you get say 30 to 40 VAC etc. If somewhere that dips severely, that's the stage losing signal.

I've had similar problems. one time I didn't have a good earth to the PI, so it was getting very low current through other means, in another I had a small solder blob touching between the tip and sleeve connections of the output jack, it wasn't a great blob, and had something like flux blocking it from being super strong, but as soon as I removed it, bam output.

~Phil

[DGNGuitars]

Thanks for the input.

Phil, since I don’t have a scope that’s a great idea!! I’ll check back in when I have some time to get to it in the next day or so.

Thanks Dan

[DGNGuitars]

and if you don't have a scope you can put your DMM to Volts AC and see what kind of voltages you get at each stage. Every amp is different, but you'll get say an input of 100 - 400 mV sine in, and at the next stage several volts ac, then at the PI output you get say 30 to 40 VAC etc. If somewhere that dips severely, that's the stage losing signal.

I've had similar problems. one time I didn't have a good earth to the PI, so it was getting very low current through other means, in another I had a small solder blob touching between the tip and sleeve connections of the output jack, it wasn't a great blob, and had something like flux blocking it from being super strong, but as soon as I removed it, bam output.

~Phil

Looks like I’m getting solid voltage readings right up to the output if the PI. This is my fender-ish clean channel that’s still wired into the circuit.

400mv 1khz sine in, volume at max.

1st Plate - 17.65 vac
2nd Plate - 45.9 vac
PI Out - 56.3/41.3 vac
OT Secondary - 3.7vac

The OT secondary measurement seems super low, right? Seems safe to assume this is an OT issue.

Thanks Dan

[pompeiisneaks]

That definitely seems low, yes. I'd look to ensure there aren't any poor connections on the output jack, and/or problems with the speaker wire or speaker etc, so as to rule out the connections themselves. (It's much more common for it to not be the OT, but doesn't mean it won't be the OT).

Double check that the inputs to the grids of the output tubes match what the output at the PI says as well. Make sure something between the PI and tubes isn't eating signal. I.e. literally touch the dmm probe to the tube pin for the grid to ensure that the same strength is coming in. Now it is a bit 'odd' that you've got a big mismatch of 56 vs 41 VAC on the output, but the only way to easily comopare that would be with a scope to see if one side is just smaller due to the PI imbalance or if it's something eating the signal via destructive feedback etc.

It does point to it being either at the PI or at the output for sure.

I just can't remember what the secondary side of the OT normally has for VAC. but 4VAC does seem low.

~Phil

[DGNGuitars]

That definitely seems low, yes. I'd look to ensure there aren't any poor connections on the output jack, and/or problems with the speaker wire or speaker etc, so as to rule out the connections themselves. (It's much more common for it to not be the OT, but doesn't mean it won't be the OT).

Double check that the inputs to the grids of the output tubes match what the output at the PI says as well. Make sure something between the PI and tubes isn't eating signal. I.e. literally touch the dmm probe to the tube pin for the grid to ensure that the same strength is coming in. Now it is a bit 'odd' that you've got a big mismatch of 56 vs 41 VAC on the output, but the only way to easily comopare that would be with a scope to see if one side is just smaller due to the PI imbalance or if it's something eating the signal via destructive feedback etc.

It does point to it being either at the PI or at the output for sure.

I just can't remember what the secondary side of the OT normally has for VAC. but 4VAC does seem low.

~Phil

So after some further searching I found a method to bench test the output transformer. I hooked up 3.15 vac from my variac to the 8 ohm tap secondary. Got a measurement of approximately 15vac on the primaries. I have another OT, 30 watt same type from Edcor also with 6.6k primary and ran the same test. On the 30 watt OT I got 90vac on the primary. Seems to me there is def a short in my 50 watt PT. What a bummer....

I really appreciate the help.
Thanks Dan

[pompeiisneaks]

Ugh that is a bummer, sounds pretty much confirmed

~Phil