Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BrianS on October 02, 2019, 12:15:51 pm
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Hello folks! I haven't posted in a long time...too busy repairing guitars and amps! I have one that is stumping me a bit here. I "inherited" this amp from another tech who had given up on it. I'm always up for learning, so here I am.
Amp: Fender Bandmaster 6G7-A (https://el34world.com/charts/Schematics/files/Fender/Fender_bandmaster_6g7a_schem.pdf)
When I received the amp, the oscillator had already been rebuilt, along with some other repairs/component replacements. I did not get to speak with the other tech, but his notes mention "circuit still does not work". The oscillator oscillated, but there was some really heavy "beating" or pulsing...getting worse and more "disjointed" as the intensity was turned up. After studying the circuit and looking at the phase splitter part, I noticed that someone had replaced on the of the 56k load resistors with a 56 OHM resistor! Obviously this unbalanced that part of the circuit.
After replacing the 56 ohm with a 56k, I now have a usable tremolo up to just past noon on the intensity control. Past that I still get some pretty bad beating or pulsing. Looking on the o-scope, it's a fairly ugly signal. The oscillator signal coming out of the phase splitter looks to be a nice sine wave, so I don't think that is where the problem lies.
I have measured the 100k 5% mixing resistors and they are within spec...at least when there is no voltage on them. I have not measured the 470k resistors in that part of the circuit.
One curious thing happens as well: when I touch my o-scope probe to the grid of the amplifier's phase splitter...after the .001uf cap, where the guitar signal comes in...most of the beating goes away until I turn the intensity knob up almost all the way (3/4 up or so). This only happens when I put the probe in that one spot in the circuit.
My thought right now is that maybe the output of the trem circuit is not completely balanced, and so there is still this pulse there that is not cancelled out and is getting amplified. I have ordered new, 1% resistors for all the mixing resistors which will get here in a couple days, but I'm afraid I'm overlooking something.
Oh, I have also replaced all the tubes in this part of the circuit...they were all inexpensive Chinese tubes...to no avail.
Any insight is appreciated. Thanks.
Brian Stewart
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> when I touch my o-scope probe to the grid of the amplifier's phase splitter...after the .001uf cap, where the guitar signal comes in...
That seriously un-biases the phase splitter. It goes "numb". Not an obvious sign of anything wrong.
Past that, I'm stumped. The usual: bad joints, drifted or wrong values, major DC voltage errors.
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Checkout this thread which I saved in PDF: especially sluckey's Reply #9: Fender Service Bulletin
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"...sluckey's Reply #9: Fender Service Bulletin"
I have that service bulletin and have used its advice to repair that style of tremolo circuit. The last one I did was indeed the lead dress.
The "harmonic tremolo" is a completely different circuit...much more complex!
I'll wait for my new parts and report back with any results. Any other advice is appreciated.
Oh...yes, all the voltages are correct. Other folks have messed with this amp, but so far, their work looks decent enough...other than the wrong value resistor...Thanks!
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since you have a scope, try lifting the "right" side of the .02 that goes to the "bottom" of the intensity pot and scope the cap to see if you have a "clean" 3-15hz kinda signal
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After replacing the 56 ohm with a 56k, I now have a usable tremolo up to just past noon on the intensity control. Past that I still get some pretty bad beating or pulsing. Looking on the o-scope, it's a fairly ugly signal. The oscillator signal coming out of the phase splitter looks to be a nice sine wave, so I don't think that is where the problem lies.
This may not be a valid conclusion, because the trem PI might malfunction if hit with too much intensity.
when I touch my o-scope probe to the grid of the amplifier's phase splitter...after the .001uf cap, where the guitar signal comes in...most of the beating goes away until I turn the intensity knob up almost all the way (3/4 up or so).
This suggests a gerry-rigged solution. What is the input (shunt) resistance of your probe + scope. Put a shunt resistor of that value at the amp's PI. And reduce the trem intensity by 25%. Problem solved?
Maybe the whole problem is excessive intensity (trem output voltage).
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"This suggests a gerry-rigged solution. What is the input (shunt) resistance of your probe + scope. Put a shunt resistor of that value at the amp's PI. And reduce the trem intensity by 25%. Problem solved?
Maybe the whole problem is excessive intensity (trem output voltage). "
I actually considered that. I did swap out the oscillator 12AX7 with a 12AY7...didn't do anything, but that was just kind of a shot in the dark. I'm not exactly sure how to measure the input resistance of the scope probe. When I measured tip to ground clip, I got an infinite reading. The scope says it has a 1 meg input impedance.
I believe I swapped out the tubes AFTER I changed out the 56 ohm resistor, so the tubes should be fine.
Thanks for the suggestions. I won't be able to get back to the amp until Monday. I'll post anything I find out. There's not a lot of info on trouble-shooting or repairing this particular circuit out here on the web.
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not a lot of info on trouble-shooting or repairing this particular circuit
If "we" folk can't get you close enough, it probably ain't never worked :laugh:
hang in there, it was free right :icon_biggrin:
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Never heard this tremolo but would like to. The you tube videos I've seen (heard) apparently leave a lot to be desired because the examples demonstrated are thumping / pulsing and don't sound nearly as good as the vintage VOX tremolo. :dontknow:
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Here's an article on probe + scope resistance (impedance): http://ecelabs.njit.edu/ece291/lab7.php (http://ecelabs.njit.edu/ece291/lab7.php) Lots of info on the web including vintage scope manuals & books free on pdf.
Anyway you already know the internal scope resistance.
Also, the Intensity pot is listed as 10M. If you bypass it -- lugs 1-3 -- with a 10M resistor, that will reduce its value to 5M (with a somewhat skewed audio curve). Anyway I think that will bleed more intensity voltage to ground. Maybe experiment with different values of bypass R's. This should reduce trem voltage intensity. This might work, provided that enough intensity remains. However, the change in intensity will happen more rapidly with pot rotation when an effectively smaller value pot is used.
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How does the circuit respond when using a low output guitar pickup such as vintage Strat or Tele? I ask because I know that modern hot pickups can overdrive the modulator tube. I fought this in a 'Revibe" that used the same modulator and have some ideas that may help. But first, play a low output guitar such as was available back in the early '60s to see how the circuit behaves.
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Update:
I changed out the "mixing" resistors (100k & 470k) with new 1% tolerance versions and that didn't change anything. At that point the trem was working perfectly up to just past noon on the intensity control. I swapped the 12AY7 back in and got usable trem up to about 7 on the dial. At that point I was thinking that maybe that is how the amp always was, as discussed earlier in this thread.I called the owner of the amp and explained the situation and he told me that that was how the amp was before the trem stopped working 100%. He was happy with the result of the repair.
I did not feel that I got 100% satisfaction out of this repair, but having never heard, played or worked on an amp with this style of tremolo circuit, I feel I didn't have a good reference point. I read through the patent papers for this circuit, which explains the function of every component, only to find that the circuit used in the amp has a bunch of extra components, including an extra tube stage.
My past experience with other tremolo circuits in other amps...amps I've either sold in my shop, played for fun, or have worked on...is that they all vary a bit in how they sound and function. Is it possible that this amp "thumped" once you turned the intensity up past noon even when it was brand new? I don't know. Anyway, it's done and the customer is satisfied, so I guess that's that. Thanks for the input.
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Is it possible
I'll make a qualified, maybe :icon_biggrin:
I believe thumping is a known thing in fender amps, even believe it's maybe a small reason Sluckey came up with his Trem-o-lator circuit :dontknow:
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Had to put the 12ax7 back in the oscillator circuit, as the 12ay7 would not keep it going at the lowest and highest speeds...
I do not like this circuit...except for the tone...it sounds great when working correctly.
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Hey
Reviving this thread.
I came across your problem because I was having the same. It is the 5 triode harmonic trem.
The pulsing and throbbing in the speaker kept getting stronger with higher intensity. And this made it useless.
My solution: I upped the 100n caps on both sides of the intensity pot to 220n.
I still have to scope the difference with 100n.
Maybe this helps, maybe it won’t , just my 2 cents . 👍🏼
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It might be that by increasing the low pass filtering, the max available modulation is being reduced. You could check for that by measuring the modulation level at max, with and without the modded values.
I suggest to consider adding a trimmer to the modulation cathodyne’s plate or cathode resistor, to enable the balance of the modulating common mode signal to be tweaked to obtain the most effective null.
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The 10M pot is unavailable in europe , and one pot from tedweber's shop would cost me 60 bucks :l2:
So I use a 3M pot in series with 3M3 resistor to ground. So in my case the low pass is a bit screwed up. half the resistance , double the caps I thought :)
However, this gives a little bit of trem at 0, and at 10 a nice deep one. So in my opinion the 10M pot isn't necessary given the cap tweaking gets rid of the wobble and the intensity sweep is pretty nice.
I cooked up a nice recipe: jim kelley front end, ,fender harmonic trem and reverb, an LTP and 4x 6v6. All of this in a vibrochamp chassis :icon_biggrin:
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I cooked up a nice recipe: jim kelley front end, ,fender harmonic trem and reverb, an LTP and 4x 6v6. All of this in a vibrochamp chassis :icon_biggrin:
That is asking for trouble. :w2:
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Explain?
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That's a lot of amp in a little chassis!
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You’d be surprised !
50w Marshall OT, big fat PT
Just stuff I had lying around 👍🏼
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the problem I've seen with big amps in small boxes, the box can't hold the power and starts tapping itself to the beat, kinda cool visual, musicians don't like it though :icon_biggrin:
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May be tangentially helpful:
I was recently tweaking my Weber ReVibe which is fender harmonic tremolo in a standalone unit.
In answer to one of my questions, Jeff Gehring who designed the ReVibe, suggesting that I could lower the value of R37 (see attached schematic). He said if I went too low, I would know because it would cause an unpleasant throbbing sound that goes in time with the speed knob.
Don't know if this helps but your question rang a bell.
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May be tangentially helpful:
I was recently tweaking my Weber ReVibe which is fender harmonic tremolo in a standalone unit.
In answer to one of my questions, Jeff Gehring who designed the ReVibe, suggesting that I could lower the value of R37 (see attached schematic). He said if I went too low, I would know because it would cause an unpleasant throbbing sound that goes in time with the speed knob.
Don't know if this helps but your question rang a bell.
Do you have a link to that thread?
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May be tangentially helpful:
I was recently tweaking my Weber ReVibe which is fender harmonic tremolo in a standalone unit.
In answer to one of my questions, Jeff Gehring who designed the ReVibe, suggesting that I could lower the value of R37 (see attached schematic). He said if I went too low, I would know because it would cause an unpleasant throbbing sound that goes in time with the speed knob.
Don't know if this helps but your question rang a bell.
Do you have a link to that thread?
Sorry I didn't see this before I posted the same thing in your thread (this time with an oscilloscope picture).
Jeff's tip wasn't in a public forum. But this is what he said: "if you want to increase the intensity you can try lowering the value of R37 to 2M or even 1M. If you go too low you will get a very unpleasant sound that is synchronous with the speed setting"