The only extra work I did while moving the PS caps back to original design, was I replaced the 0.05uf oil cap and I resoldered a 100k resistor off V1 which had seemed to be soldered badly. When I replaced the electrolytics, I soldered them in exactly how the originals were soldered without looking at the schematic. So yes I confirm that the negative side of the 10uf cap is grounded
Jimmy - My suggestion to reduce the value of the first cap was based on the data sheet for the tube. I posted a link to it in the post. Tube manuals, and the data sheets they contain, are published by the engineers that designed the tubes. That's where the 10uF recommendation comes from, I didn't just make it up. The other posters on this thread don't seem to care much about the information contained in the tube manual.
Here's a link to a compilation of various tube manuals: Frank's electron Tube Data sheets
I was curious about the ability to heat the 6X4 and the other tubes in the amp with the same heater winding because I had never seen that before - typically, rectifiers have their own heater winding. So I asked about it over on another forum. Mostly, I was curious about the heater thing but there was also some discussion about the capacitance issue. It was pointed out to me that it is possible to use more capacitance, but only if the current demands of the circuit are low enough. That is actually mentioned in the data sheet, which I didn't notice. Since the schematic has no voltages listed on it at all, it's not obvious how much current is being drawn. As I mentioned, I saw some similar amps that used 20uF as a first cap. Your's is the only one I've seen with 40uF, which I still think is too much. The comment by FenderLover (who is a professional guitar amp tech) was that "40uF is going to kill the 6X4".
Here's a link to that thread: https://www.audioasylum.com/cgi/t.mpl?f=tubes&m=272529
While hum can be due to not enough filtering capacitance, it should be noted that there are other possibilities, one of which is heater-cathode leakage / shorting. One of the comments in that thread noted that 6X4s often exhibit heater-cathode problems when they are heated with a supply that is grounded on one end - as it is in your amp.
Have you tested any of the tubes in your amp? It's always a good idea to make sure your tubes are good, not necessarily new but problem free, before you start troubleshooting the circuit. If your tubes are all OK, then my wild-***-guess would be that the squealing might be related to the tremolo section. I'd try to take it out of the circuit and see if the squeal is still there.
When this thread was moved to the Instruments and Amps Forum, my comment was that it had been sent into "exile". The section where it was originally has many really sharp regular posters. Often, the discussion there is waaay over my head and extremely technical in nature. It's been my experience that when threads are moved over to this forum, there are fewer knowledgeable people involved. Myself included, probably. I'm a hobbyist and most of my experience is with vintage stereo although I do some DIY building and have a little guitar amp experience.
I'd suggest you try posting about your amp over on the Vintage forum of AA: https://www.audioasylum.com/cgi/tv.mpl?forum=vintage
They don't send posts about guitar amps into "exile" and there are some folks there, like FenderLover and others, who have a lot of experience with guitar amps. Nothing wrong with posting about issues on multiple forums. You will get a broader range of expertise by doing so.
Here's a link to a compilation of various tube manuals: Frank's electron Tube Data sheets
I was curious about the ability to heat the 6X4 and the other tubes in the amp with the same heater winding because I had never seen that before - typically, rectifiers have their own heater winding. So I asked about it over on another forum. Mostly, I was curious about the heater thing but there was also some discussion about the capacitance issue. It was pointed out to me that it is possible to use more capacitance, but only if the current demands of the circuit are low enough. That is actually mentioned in the data sheet, which I didn't notice. Since the schematic has no voltages listed on it at all, it's not obvious how much current is being drawn. As I mentioned, I saw some similar amps that used 20uF as a first cap. Your's is the only one I've seen with 40uF, which I still think is too much. The comment by FenderLover (who is a professional guitar amp tech) was that "40uF is going to kill the 6X4".
Here's a link to that thread: https://www.audioasylum.com/cgi/t.mpl?f=tubes&m=272529
While hum can be due to not enough filtering capacitance, it should be noted that there are other possibilities, one of which is heater-cathode leakage / shorting. One of the comments in that thread noted that 6X4s often exhibit heater-cathode problems when they are heated with a supply that is grounded on one end - as it is in your amp.
Have you tested any of the tubes in your amp? It's always a good idea to make sure your tubes are good, not necessarily new but problem free, before you start troubleshooting the circuit. If your tubes are all OK, then my wild-***-guess would be that the squealing might be related to the tremolo section. I'd try to take it out of the circuit and see if the squeal is still there.
When this thread was moved to the Instruments and Amps Forum, my comment was that it had been sent into "exile". The section where it was originally has many really sharp regular posters. Often, the discussion there is waaay over my head and extremely technical in nature. It's been my experience that when threads are moved over to this forum, there are fewer knowledgeable people involved. Myself included, probably. I'm a hobbyist and most of my experience is with vintage stereo although I do some DIY building and have a little guitar amp experience.
I'd suggest you try posting about your amp over on the Vintage forum of AA: https://www.audioasylum.com/cgi/tv.mpl?forum=vintage
They don't send posts about guitar amps into "exile" and there are some folks there, like FenderLover and others, who have a lot of experience with guitar amps. Nothing wrong with posting about issues on multiple forums. You will get a broader range of expertise by doing so.
I have also worked professionally for some years as a guitar amp designer (most folks here will recognize the name of the companies I worked with). I would certainly be the last person to advocate putting an excessively large capacitor on s vacuum state rectifier. At the same time I have come across many single-ended guitar amps that suffer from hum due to inadequate filtering. Basically, the fact is that you *will* get hum with a single 10uF or 20uF filter cap - guranteed. In the example of this amp, I would recommend retrofitting a Pi filter arrangement, with a 20 or 10uF first cap, then a 47ohm resistor followed by a 47uF or 100uF cap feeding the OT. This is 100% guranteed to be effective.
Sent from my phone. Please excuse any typpos.
Sent from my phone. Please excuse any typpos.
OK, I see your earlier suggestion of using a 100uF cap was made before you noticed that this amp uses tube rectification. If you go back a bit he posted a schematic.
I agree wholeheartedly with your revised suggestion, though. I thought about suggesting the addition of an RC section earlier as a means to lower PS voltages but I figured it best to let him get the amp operating correctly before suggesting any major mods. Switching the 40uF and one of the 10uF seemed a lot simpler as a means to protect the 6X4 from damage, although it may already be damaged. He hasn't said if he has tested any of the tubes and until they are confirmed as being good it can be difficult to chase down other potential problems.
I guess my suggestion would be to 1) test all the tubes 2) replace all the grey oil caps, which are known to be problematic 3) make sure all other parts are in spec before going further. Just standard procedures when restoring vintage gear. Unfortunately, the schematic doesn't show any voltages. I'll have to check to see if the similar schematics I have list voltages.
Dealing with hum in a guitar amp is a bit of a balancing act though because PS sag is often considered desirable and making the PS too stiff will reduce or eliminate the sag. Vintage guitar amps do have some hum but most players are willing to put up with some level of hum at idle because they like the effect of PS sag while the amp is being played. Obviously, there shouldn't be seriously audible hum while the amp is being played.
Since you seem to know your way around guitar amps, do you have any suggestions on what might be causing the squeal he mentions? That seems to be his major concern, not hum.
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Hi folks....ok the preamps are new tungsols, the 6x4 is NOS, the technician who tried to fix it previously just replaced the tubes and used a JJEL84 to replace the old 7189A (unmarked tube) which I am using now to do the tests. I am guessing that the 7189A tube is functioning because the second last test I did (when I was getting hum) there was actual guitar signal going through and also volume and reverb were working. The squeal which is now present doesn't allow me to keep the amp on for more than a couple of seconds. I'll try and figure out exactly which resistor I resoldered and point it out to you as to the schematic.
Often it's caused by one side of the OT secondary not being grounded (even when there's no NFB). The other common reason is poor lead dress in the preamp.
My comments about power supply filtering still need to be born in mind regardless, as otherwise the amp will still suffer from 120/100 Hz hum when the volume is turned up. It's a fact I can't change.
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Well I tried placing the old 6X4 in and the squeal changed behavior, it went from very high pitch and rapidly fell to the squeal that I have with the other 6X4. I also noticed that there is no low hum (from what I can hear). It was really obvious when I had swapped the 40uf and the 10uf PS caps around. Also when pulling the 12AX7 right next to the 7189A tube, I don't get any squeal at all.
I also found this site for further info:
https://bensaudioprojects.wordpress.com/2012/08/06/teisco-10-renovation-and-combo-head-conversion/
I also found this site for further info:
https://bensaudioprojects.wordpress.com/2012/08/06/teisco-10-renovation-and-combo-head-conversion/
I suspect you are getting a different DC voltage from the old 6X4 relative to the others, which is now probably suffering from poor emission.
I also tend to think that your amp suffers from poor layout and lead dress, so that you're getting positive feedback between wires. What happens when you remove one 12AX7 somewhat points to that. In the article you posted the link to, the amp's layout is referred to as "spaghetti", as as shown in thee pictures
https://bensaudioprojects.files.wordpress.com/2012/08/teisco_guts.png
https://bensaudioprojects.files.wordpress.com/2012/08/collected-grounds.png
If your amp looks anything like that one, then I hate to say it really needs to be taken apart and rebuilt cleanly. 🙁
Have you measured all the resistors to see if they are in spec? You might need to desolder one side in some cases in order to get an accurate measurement.
I still suspect the tremolo section might be contributing to the squeal problem. I'd suggest you rebuild it completely - replace all the caps involved (the .02 and two .01), if you haven't already, and check the resistors. Don't forget to check the 1M pot that controls the tremolo to see if it's working correctly. If the tremolo section is the source of the squeal, pulling the 12AX7 next to the 7189A would eliminate it, just as you described.
If you want to check voltages without listening to the squeal, just connect a dummy load resistor to the output instead of the speaker. If the speaker is 8 ohm use an 8 ohm power resistor that's rated at 20w or so to be safe. Or combine two 10w resistors. Exact values are probably not too critical for a quick voltage check. Once you get some voltage readings you can determine just how hard the output tube is being run - I'd suggest getting readings at both 117v with a variac and at whatever wall voltage you have.
Eventually, you really should add an R-C section to the power supply. Doing so will eliminate the stress on the 6X4, take care of the hum, and bring the voltages down a bit so the tube heaters aren't too high and the output tube isn't being pushed too hard.
I still suspect the tremolo section might be contributing to the squeal problem. I'd suggest you rebuild it completely - replace all the caps involved (the .02 and two .01), if you haven't already, and check the resistors. Don't forget to check the 1M pot that controls the tremolo to see if it's working correctly. If the tremolo section is the source of the squeal, pulling the 12AX7 next to the 7189A would eliminate it, just as you described.
If you want to check voltages without listening to the squeal, just connect a dummy load resistor to the output instead of the speaker. If the speaker is 8 ohm use an 8 ohm power resistor that's rated at 20w or so to be safe. Or combine two 10w resistors. Exact values are probably not too critical for a quick voltage check. Once you get some voltage readings you can determine just how hard the output tube is being run - I'd suggest getting readings at both 117v with a variac and at whatever wall voltage you have.
Eventually, you really should add an R-C section to the power supply. Doing so will eliminate the stress on the 6X4, take care of the hum, and bring the voltages down a bit so the tube heaters aren't too high and the output tube isn't being pushed too hard.
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Ooops . . . no it won't affect the heater voltage but once you've measured it while using wall voltage you can figure out if you need to add a low value resistor to bring it down.
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