Hey everyone I'm trying to get this Hot Rod Deluxe back in working order. I cleaned it up a bit to the point where the output doesn't fizzle out. I think the power tube sockets were a bit oxidized. However, the output is still pretty quiet and it may be a bit more distorted than it's supposed to be (or at least the distortion isn't as smooth as it should be). You can hear it here (volume is at 5 or 6, in other words right at noon where it should be getting pretty gritty anyway).
Going off of this service manual, injecting a 1kHz tone at the power amp input and calibrating it to 750mV at TP23, the signal is still clean at the output of the 12AX7 phase inverter (TP24 and 25) but then it gets ugly on the output of the 6L6 stage (TP27 and 28). Here is a scope of that. Looks the same on both test points. Is it safe to say the issue is in the 6L6 stage? All my DC voltages look ok and from what I can tell the tubes aren't drawing an excess of current. Got them biased comfortably around 60 mA.
There are some signs of charring, including the 5W resistors between the 48v rails and the 16v rails, but the 16v rails look clean. Probably going to replace them to be safe, but I don't suspect they are causing the issue.
Anyone have any suggestions on where to look next? I'm ordering a pair of 6L6s to test it with, and I'll probably do other regular maintenance like filter caps and such. Should I pull the grid resistors and test them?
Going off of this service manual, injecting a 1kHz tone at the power amp input and calibrating it to 750mV at TP23, the signal is still clean at the output of the 12AX7 phase inverter (TP24 and 25) but then it gets ugly on the output of the 6L6 stage (TP27 and 28). Here is a scope of that. Looks the same on both test points. Is it safe to say the issue is in the 6L6 stage? All my DC voltages look ok and from what I can tell the tubes aren't drawing an excess of current. Got them biased comfortably around 60 mA.
There are some signs of charring, including the 5W resistors between the 48v rails and the 16v rails, but the 16v rails look clean. Probably going to replace them to be safe, but I don't suspect they are causing the issue.
Anyone have any suggestions on where to look next? I'm ordering a pair of 6L6s to test it with, and I'll probably do other regular maintenance like filter caps and such. Should I pull the grid resistors and test them?
First, check a couple things. Look VERY closely at the solder on the two 6L6 socket pins. I find that just about every HR series amp that comes through my shop has cracks there. In fact, I just automatically resolder the two power tube sockets once the amp is open.
Next, look at the phase inverter tube, next to the power tubes. Is ther good B+ on BOTH pins 1 and 6? Too many of the HR series likes to open a phase inverter resistor. That would cause a lack of B+. It also causes a lack of part of your output waveform.
Next, look at the phase inverter tube, next to the power tubes. Is ther good B+ on BOTH pins 1 and 6? Too many of the HR series likes to open a phase inverter resistor. That would cause a lack of B+. It also causes a lack of part of your output waveform.
More than likely the output transformer has shorted turns if the cathode current and control grid drive is correct on the 6L6GCs.
Also, make sure your speaker is plugged into the MAIN speaker jack, not the extension speaker jack. The main jack has a shorting contact, so if you plug into the extension the output remains shorted across. Also make sure the main jack contacts are not bent so as to short against the shaft of the plug.
Didn't see any shorts on the output transformer, that was one of the things I suspected.
Cool I'll do that the next chance I get.
I'll check that too.
Even if the waveform looks clean after the phase inverter?
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You can't test for that with a meter. A shorted transformer isn't going to short end to end for zero ohms, it would short turn to turn, and that would reduce the resistance by half an ohm or something. Plus it would require you already know exactly what it originally measured. Not only that, sometime the short is really an arc and it only happens at high voltage, but won't at the volt your meter uses.
Go to the Geofex web site, and look at RG Keen's cool transformer tester in the tech tips.
I looked at the datasheet and it lists DC resistances. The primary windings look spot on but the "8 ohm" secondary is reading about half of its rated value.
To be sure, I'm gonna put together that test rig. Is the 6V supply a specific value? Or would 9 or 5 work fine?
Check the 6L6 screen resistors R61 and R62 for open circuits and values going way high in resistance.
Failure of these is common too.
If they need replacing then up their values and power, Use 1K 5W resistors for replacements.
Cheers,
Ian
Failure of these is common too.
If they need replacing then up their values and power, Use 1K 5W resistors for replacements.
Cheers,
Ian
Do you have a speaker connected? That will lower the resistance reading. In any case it is HIGHLY unlikely your secondary has shorted turns.
They both read close to their nominal value.
Nope I disconnected the speaker.
Good to know. I'm really at a loss as to what else if could be though.
So troubleshoot. Don't think in terms of "maybe it is this? Maybe it is that?" Apply a steady signal to the amp. If you have a generator, fine, but I just use a CD player in my shop. Easier to listen to than a sine wave.
Then trace it through the amp. WHERE is the signal being diminished?
Then trace it through the amp. WHERE is the signal being diminished?
I did pass a tone through it before I posted. Saw the signal degrade at the plate of the power tubes. Looked like a clean sine at the screen grid. I linked to a photo of the oscilloscope in the OP. The signal looks the same on both tubes and on the OT secondary (scaled of course).
Edit: I can check all the voltages on the tube pins again to be sure they're ok.
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What is the amplitude of signal at the power tube grids?
Transformer failure is not common, but test the output transformer for shorted turns. GO to RG Keen's Geofex site, and find the simple transistor test under tech tips.
Transformer failure is not common, but test the output transformer for shorted turns. GO to RG Keen's Geofex site, and find the simple transistor test under tech tips.
With a loosely calibrated test tone, it’s right around 10-11 volts where it should be. And at the plate it’s a distorted 15v peak.
Plate (DC) voltages are 425
Control grid voltages are -44.7
Screen grid voltages are 424
Two things come to mind. One is to pull the OT secondary wires off the jacks and clip a speaker directly to them. ANy different?
And then do the transformer test I cited a couple posts ago.
And then do the transformer test I cited a couple posts ago.
Yes! Much louder! Still sounds nasty but I’m 99% sure if’s because of the alligator clip jumpers I’m using.
What does this mean? Loose connections or oxidation on the jack? Is the best bet to swap the jack out?
Edit: Just to clarify, by “nasty” I mean it sounds a little scratchy since the alligator clip isn’t making a great contact. Not distorted like it was before.
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Means the jack is shorted.
There are two speaker jacks, the main one is a shorting jack. We normally plug the internal speaker into that jack. The other jack is the extension speaker jack. it is wired parallel the main, but lacks a shorting contact. One of the most common problems I see is someone plugged their internal speaker into the wrong jack, leaving the shorting jack across the output.
SO if clip wires sounds full, then either your main jack was shorted, or you had the speaker in the wrong jack to start with.
There are two speaker jacks, the main one is a shorting jack. We normally plug the internal speaker into that jack. The other jack is the extension speaker jack. it is wired parallel the main, but lacks a shorting contact. One of the most common problems I see is someone plugged their internal speaker into the wrong jack, leaving the shorting jack across the output.
SO if clip wires sounds full, then either your main jack was shorted, or you had the speaker in the wrong jack to start with.
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