Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: GaryE on February 20, 2019, 12:06:02 pm
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Hello everyone. Long time lurker, first time poster. I have just finished my Revibe unit using Sluckey's modded layout. Thank you to Sluckey and Doug for al the fantastic info BTW.I used Hammond transformers as I am a proud Canadian(please don't hold that against me!) The PT is the 261D6 ,a 194A choke and the venerable 1750A reverb trans. I used a MOD 4ab3c1b tank but have the 9ab3c1b 3 spring tank on the way,to see which I like best.Anyway ,so far the reverb sounds pretty much like the 6g15 clones I have built before and the vibrato circuit is excellent ! I have repaired a few of the brown Fender amps with this circuit and found it to be cool so this is pitch perfect for my needs.However ,there is a slight issue . When I crank the speed and intensity levels up past 8 or 9 the vibrato disappears .I used a pair of my strongest 12ax7s in this section as I know that can be a problem in the 6g12 circuits. If I turn the controls back to a lower setting the effect slowly reappears. I did notice that using this 261d6 trans supplies about 10-15 volts less DC across the board compared to Sluckey's posted voltages. I tried using a 220k resistor in place of the 100k on the intensity pot and that seemed to shift the point of diminishing effect a bit but did not completely cure the problem. Any insight would be a great help! Thanks
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When I crank the speed and intensity levels up past 8 or 9 the vibrato disappears
Is the LED still blinking when the vibrato disappears?
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No ,It stops blinkingbut stays lit.
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Replace the LED and see if that helps.
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Thanks! I will try that tomorrow and see .
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Ok, tomorrow was actually yesterday but I finally got around to changing the LED .It may have slightly changed the cutoff point where the vibrato stops oscillating but only a little .This seems to be a problem only on the fastest setting with the range switch set on the faster mode.I also forgot to note that I didn't have a 5.1 meg resistor for the speed pot so I substituted a pair of resistors to make it 5.5meg. I didn't really see tat as an issue but maybe that's part of the problem?
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> I didn't have a 5.1 meg resistor for the speed pot so I substituted a pair of resistors to make it 5.5meg. I didn't really see tat as an issue
I think it might be. Draw-up exactly what you have done.
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Can your meter measure the frequency of the trem signal? Needs to read between 3 to 10Hz. My Fluke will read that low. Don't know about others. I put the 5.1M across the speed pot because my oscillator would drop out below about 3Hz. And my oscillator would also drop out around 10Hz.
If you can get 3.5Hz to 9Hz I'd call it golden. The three caps and resistors between the plate and grid control the frequency. Play around with them and I bet you will get a usable range.
Since you're concerned about the high speed dropping out, increase the 100K to limit the high end.
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Yeah, my meter has a HZ setting. Never actually used it. Where do I measure that?😄
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Look at my schematic, especially the green notes.
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Got it. I get a reading of 5.3 HZ Which according to your guidance is well in the ballpark.
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OK. Now turn the speed up until the signal drops out. What is the highest frequency you get? Now turn the speed down until the signal drops out. What is the lowest frequency you get? What is the AC rms voltage at your highest and lowest speed. Measure all this stuff at the top of the intensity pot or footswitch jack.
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OK I get 2.9HZ at the lowest speed
5.5HZ at the highest speed as the effect starts diminishing
I'm a little confused by the AC RMS voltages
1.4 VAC at the lowest speed with the intensity at max CCW
0.72VAC at the highest ,any speed higher sends the voltage down to practically zero.
Does this make sense to you?
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Yes it makes sense. But I was expecting the voltage to not drop off until around 10HZ. Are you using a red led? What is the dc voltage on the osc. tube cathode?
Maybe go back to the stock 4.7K and 22µF on the tube cathode.
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OK. I put the 4.7/22uf pair in.The HZ range goes from 3.4HZ to 6.3HZ low to high speed.The effect strengh remains way more constant through the range of speeds all the way to full CW,However the intensity of the vibrato effect seems less all the way round.I definitely prefer the tone (and the visual) with the LED in place
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Red face embarassment time. I put the LED back in place and then did some tube rolling. I found my best Mullard 12ax7 and put it in V5 position . Voila! The vbrato effect stays on throughout the entire speed range. The HZ figures are 2.94 to 5.85 HZ low speed to high.Rookie mistake!