Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: boogieWoogie on February 17, 2019, 06:05:18 pm
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Hi folks,
I've built the Hoffmann Revibe. Reverb side is working well. The tremolo is ticking / motorboating badly. I'm able to disconnect the sections to debug, but I'm not sure what to look for. I've spent ~10 hrs double checking for a wrong component, but to no avail. A few questions:
1) Should the "upper" LFO stage produce a sine wave output (i.e. plate of V5-A should show a sine wave?)
2) Should the "lower" LFO stage produce a sine wave output (i.e. plate of V5-B should show a sine wave?)
3) My B+ voltage (point D) is 344V. Is this too high? (Variac doesn't help the ticking).
4) The upper tube-stage (V5-A) looks like a LFO. What's the lower one (V5-B) for? If it's an oscillator too, how should it complement the upper LFO (V5-A)?
I've got a scope & can post photos of traces if that would help.
Thanks in advance for any advice,
BW
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Look at page 11 of this pdf for some theory on that trem circuit...
http://sluckeyamps.com/misc/Amp_Scrapbook.pdf
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Thanks for the great doc! From what I understand, one of the oscillator triodes should produce a nice sine wave, and the other triode should receive that sine wave at its gate & produce it's complement (180 degrees out of phase).
So my plan of attack should be to isolate the oscillator triode & rebuild it until it produces the sine wave.
Will repost if this pans out. Thanks!
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Hi Folks,
I isolated my oscillator circuit & believe I've found the problem: too much HT voltage.
I'm using Hammond power transformer 261G6 which has very similar specs to the Mojo779 in Hoffman's BOM.
(https://i.imgur.com/z7lz1Lw.png)
I used a variac to reduce the wall-voltage. When the HT voltage at point D is between ~75V & 206V, I get the desired sine wave at the plate of the oscillator tube (as shown on scope). (Variac says 75V):
(https://i.imgur.com/xNQn4LA.jpg)
When I increase the HT voltage at point D to 232V, the sine wave starts collapsing:
(https://i.imgur.com/4gnwQpN.jpg)
By 246V, even worse:
(https://i.imgur.com/JVVJvJF.jpg)
By 264V, sine wave's completely gone, and Variac is still set to less than 115V:
(https://i.imgur.com/Vu5I605.jpg)
Some possibilities:
1) I'm wrong that the Hammond specs are similar to the Mojo779, and therefore my voltages are way too high
2) One of my power-dropping resistors is bad or wrong value so my HT @ point D is way too high
Any ideas? Do folks have any voltage references I can compare to? (I've compared to the Fender diagram in Sluckey's doc & my voltages are much higher than Fender's.)
Thanks!
BW
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Here's my schematic with voltages. My PT puts out 260VAC to the FWB.
http://sluckeyamps.com/revibe/revibe.pdf
IMO using that variac to reduce line voltage to the unit is not a good way to test because you are also reducing the filament voltage.
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Sluckey, thanks for your schematic with voltages. I started checking DC voltages & could see that my HT voltage was good (~350V), but the plate voltage for the inverter tube was about 320V (should have been near 228V). Similarly, the Vcathode = 0.3V (should have been 1.5V). Clearly the tube wasn't biased right / drawing much current. While moving around some wiring to the socket, I noticed the motorboating would disappear entirely if certain long wires were located "just so", but it also seemed socket-related (flexing the wires at the socket pin would fix the motorboating). I rewired those long wires, shortening them by 1/2 in the process, and the motorboating went away. I'm thinking it was long wires, not a bad solder joint at the socket, but it's impossible to say.
Revibe works now. I would like a louder tremolo effect, and there's a bit of noise on the whole that's dissatisfying, & tone knob for verb doesn't seem to do anything, but I was testing w/ a bass guitar. Will reassess w/ a git after I catch my breath.
Thx again for the assistance!
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but the plate voltage for the inverter tube was about 320V (should have been near 228V). Similarly, the Vcathode = 0.3V (should have been 1.5V). Clearly the tube wasn't biased right / drawing much current.
High plate voltage and low cathode voltage are clues that the tube is not conducting much (very low current), rather than drawing much current. Do these voltages look more normal now that the trem is working?
The reverb tone may just be due to using a bass. The trem effect should be very strong. Listen to Hoffman's demo...
http://www.el34world.com/Misc/Music/files/ReVibeTest1.mp3
http://www.el34world.com/Misc/Music/files/ReVibeTest2.mp3
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Sorry, I meant "not drawing much current". Indeed the tube was almost off. Fixed now though (via the rewiring / shortening of wires mentioned before). Voltages look correct. My sound is very similar to Hoffman's mp3s, but his tremolo is about 15% stronger than mine. He can get his to really "bottom out" (i.e. momentarily completely shut off the signal), which is what I want.
If I were to tweak things to get more strength from the tremolo, where should I focus my efforts? A larger oscillator amplitude? I'd guess that more gain on that stage would increase the amplitude, and I'd guess that I'd accomplish that by changing the cathode resistor.
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Try this... Replace the 4.7K/22µF on V5 pin 3 (cathode) with a 5mm red LED. The LED cathode goes to ground. This will significantly increase the oscillator signal. For kicks since you have a scope, look at the trem signal now and compare the amplitude after you do the LED mod.
I did this on my revibe. I also mounted the LED in a bezel on the front panel for some eye candy. You can see my revibe here...
http://sluckeyamps.com/revibe/revibe.htm
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Neat idea. I ordered some 5mm LEDs & bezels on ebay. Hope to have results by end of weekend. I believe my Vcathode = ~1.3V so an LED Vf of 1.8 to 2.0V should give a nice goosing to the oscillator. I'll scope the waveforms before & after.
The noise I complained about wasn't noticeable w/ a guitar (humbuckers). I guess the bass had some very noisy pickups.
The tone knob does work (attenuating vs. allowing highs is what it seems to do), but could perhaps be a bit more powerful. I may put some different cap-values in there.
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I got a lifetime supply of LEDs (several different colors) and bezels from eBay too. Here's an alternate tone control which is used in many amps. It works better than that simple treble bleed tone control. Sounds good in an amp but I don't know if it will actually benefit the revibe. Easy to try though. Notice that the tone control is 500K.
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Replacing the oscillator's cathode bias resistor & cap with a red LED did not change the sound noticably. I forgot to scope the oscillator signal at plate (pin 1 of V5-A) before installing the LED. After installing the red LED, the signal at plate was an enormous 160V, peak-to-peak.
(https://i.imgur.com/SM9rayV.jpg)
This signal is somehow extremely attenuated to 2.84Vp-p (& the DC offset is removed by blocking cap C26) by the time it reaches the upper leg of intensity pot R38 (10Meg, RA).
(https://i.imgur.com/S68lARb.jpg)
(edit: no idea why this image won't render ^^, but you can copy/paste it to browser).
edit...fixed image tag...sluckey
I couldn't explain why it's so attenuated, but I wanted a larger signal so I started reducing series resistors to reduce the voltage-divider effect. I reduced R37, R39, & R34. I was able to produce a sine-wave at the gates of V4 that was quite large (3.5Vp-p). I thought that would have been enough to get the "hard-off" tremolo sound I desired, but it still sounded ~identical to the stock Hoffman build.
While telling my bandmate about this, he said, "ah you want that helicopter tremolo". He said he'd owned a tremolo pedal which had a square wave oscillator that created this sound. I watched a youtube video (), and sure enough, the square wave produces that hard-off, brutal, helicoptering tremolo I'm looking for.
Should I install some clipping diodes to clip off the peaks of the sine wave, approximating a square wave?
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Set your frequency (speed) to 5HZ and recheck the voltage at the top of the intensity pot. I have 6.3VACrms (17.8Vpp) with SPEED set for 5Hz and INTENSITY set max CCW.
I would not mess with any clipping diodes. I like the smooth vibrato effect.
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Thx for the reference points. Photo below is a bit hard to read. Upper, blue waveform is Ch 2, the oscillator's plate (pin 1 of V5-A, Hoffman's schematic). Lower, yellow waveform & yellow measurements is Ch1, non-grounded (aka. upper) end of the intensity pot (R38), w/ pot cranked CCW (which is where I'd set it for LEAST tremolo effect*). Freq = 5.05Hz
(https://i.imgur.com/r3TF4xg.jpg)
My signal at the intensity pot is much smaller than yours.
* the pot is a "reverse taper", and I admit I have trouble understanding why a standard audio pot wired w/ legs reversed wouldn't result in the same thing... and that confusion has been in the back of my mind saying, "could all of this be caused by you miswiring that reverse pot you don't understand?". This was strengthed by you saying that CCW would be max intensity (which is the opposite of what I experience). If I was to crank the pot CW, the waveform in the scope photo would be smaller, probably b/c it's now seeing the load of the rest of the circuit.
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This was strengthed by you saying that CCW would be max intensity
That's not what I said. This is what I said...
INTENSITY set max CCW
That would be zero to a guitpicker.
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Gotcha on the pot lingo.
I'm experiencing more attenuation than your design. I'll try using your values for C18 & C27 and see if my signal increases in amplitude. R37 has been verified already, but I'll replace C26 while I'm at it, just in case.
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Look at the list of changes on my schematic...
http://sluckeyamps.com/revibe/revibe.pdf
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Sure enough, the info about attenuation was in your mod notes. When I switched out C18 & C27 to use your values, I get a large sinewave at the top of the intensity pot, and the tremolo sounds more powerful.
(https://i.imgur.com/88ZzXUi.jpg)
I ended up messing around w/ diodes last weekend... it worked well at lower speeds: the tremolo could cut the signal off for longer periods of time, giving that deep helicopter tremolo I wanted, but at faster speeds, motorboating could be heard in the speaker, even if not playing. So I put a series cap between the diodes and the gate of V4-A, but that dimished the squarish wave back to a sort-of sinewave, reducing the effect of the helicopter. This tremolo wasn't designed to be a helicopter tremolo, so I'll stop trying to push square peg into a round hole.
I'll probably apply a few more of Sluckey's mods, particularly the one focused on unity gain & will report here if so.
Thx for the help w/ this.
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The only other Sluckey mod I applied was replacing R63 w/ a 22k ohm resistor. As advertised, this allowed unity gain for trem channel. It did introduce more distortion, maybe b/c I was now feeding a larger signal on to my amplifier input. I liked it, but if you're looking for a clean sound, you may want to apply Sluckey's other mods (listed on his schematic) which speak to reducing distortion.