Well, the tweeters died (square wave input) but also the power supply had overheating problems (It was a Yamaha 1000NS).
The voltage across the upper plate resistor R43 is 358-262=96V, so the plate current is 96/82=1.17mA.
The schematic shows 17.4V across the cathode resistors R37+R39=39.8K, so the current of BOTH V6A and V6B would be 17.4/39.8= .44mA, so about .22mA/tube: WAY TOO LOW.
Your finding of 71V gives about .9mA/tube, which is about right.
Don't assume whoever publishes schematics is always sober. 🙂
The schematic shows 17.4V across the cathode resistors R37+R39=39.8K, so the current of BOTH V6A and V6B would be 17.4/39.8= .44mA, so about .22mA/tube: WAY TOO LOW.
Your finding of 71V gives about .9mA/tube, which is about right.
Don't assume whoever publishes schematics is always sober. 🙂
haha, yes, I'm begining to think that!
Re test signal comments - schematic says 82mVrms which is what it was.
In fact, to test the LTP specifically I adjusted the volume so that I have the 720mVrms going into C17 (far left of the pic I posted)
The more I read on this amp adn problems others have had which are similar, the conclusion I've come to is that the circuit is flawed.
My plan is to put in a voltage divider to take the input into the LTP down to the 5.2Vp-p as well as convert the LTP to the AA964 spec with NFB and 12AT7. At least, that's my first plan!
Thanks all
P
I wouldn't worry too much: guitar amps are way more art than science. If they sound good and don't combust, then they're good amps. Most of them are based on the 5F6A (Bassman) anyway, and much have been written about these, up to the thesis level.
In order to bring them to the next level, I'd look into those Holy Grails such as the Dumble.
In order to bring them to the next level, I'd look into those Holy Grails such as the Dumble.
Please post a complete and readable schematic (or at least a link), allowing to see schematic notes.
I don't think there's anything wrong with the amp. Nothing special about the circuit as far as I can see.
There could be schematic errors, though.
I don't think there's anything wrong with the amp. Nothing special about the circuit as far as I can see.
There could be schematic errors, though.
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https://el34world.com/charts/Schematics/files/Fender/Fender_custom_vibrolux_manual.pdf
schematic in the manual.
schematic in the manual.
Thanks.
The TP DC voltage is obviously wrong, probably should read 71.4V instead of 17.4V.
Did you follow the schematic notes?
I.e.:
The TP DC voltage is obviously wrong, probably should read 71.4V instead of 17.4V.
Did you follow the schematic notes?
I.e.:
- feed a 1kHz (frequency matters) sine signal of 83mVrms to the grid of V1A,
- use an RMS meter having an AC input impedance of at least 1M (and having a bandwidth >>1kHz),
- use the controls' settings prescribed,
- use an X10 probe with the scope for peak to peak measurements?
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83mVrms for almost full power ... and with volume at half , normal for a guitar amplifier 🙂
All the small tubes in the signal path are 12AX7 , no negative feedback
The coupling cap in the phase splitter is 500pF , serious attenuation is already happening there at 1KHz and lower frequencies .
So it is used for the original purpose or not ?
For hifi many , many modifications must be made ...
All the small tubes in the signal path are 12AX7 , no negative feedback
The coupling cap in the phase splitter is 500pF , serious attenuation is already happening there at 1KHz and lower frequencies .
So it is used for the original purpose or not ?
For hifi many , many modifications must be made ...
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"For hifi many , many modifications must be made ..."
Yes,
1) throw amp in trash
2) buy a decent amp
Yes,
1) throw amp in trash
2) buy a decent amp
Fully agree.
But for guitars, the Fender Bassman 5F6A is the Holy Scripture on which nearly every other amps are based: HiWatt, Marshall, Mesa Boogie, Vox..., etc, etc.
Creation and reproduction are 2 very different things.
But for guitars, the Fender Bassman 5F6A is the Holy Scripture on which nearly every other amps are based: HiWatt, Marshall, Mesa Boogie, Vox..., etc, etc.
Creation and reproduction are 2 very different things.
The diagram is flawed. It's not 5.2Vpk-pk, it's supposed to be 5.2Vrms; your measurements are spot on near as dammit. Whoever drew it had a hangover and randomly put pk-pk around the LTP but nowhere else, and also labelled the tail voltage wrong. Your circuit works fine.
@Merlinb :
Yes, 5.2Vrms makes much more sense in the context of other preamp levels.
But it's not consistent with a plate signal of 33.5Vrms.
Now 33.5Vrms looks correct as the level needed at power tube grids for full output.
This LTPI should have a voltage gain around 25 from grid to plate.
(here's a good online LTPI calculator: https://www.ampbooks.com/mobile/amplifier-calculators/long-tailed-pair/calculator/ )
So for 33.5Vrms at the plate it takes only around 1.3Vrms at the grid.
A PI grid signal of 5.2Vrms would heavily overdrive the power amp.
Yes, 5.2Vrms makes much more sense in the context of other preamp levels.
But it's not consistent with a plate signal of 33.5Vrms.
Now 33.5Vrms looks correct as the level needed at power tube grids for full output.
This LTPI should have a voltage gain around 25 from grid to plate.
(here's a good online LTPI calculator: https://www.ampbooks.com/mobile/amplifier-calculators/long-tailed-pair/calculator/ )
So for 33.5Vrms at the plate it takes only around 1.3Vrms at the grid.
A PI grid signal of 5.2Vrms would heavily overdrive the power amp.
There is local negative feedback , as the grid resistors are not connected to ground but before the tail resistor .
So at the grid there is much less signal than roughly half that would be from 1M + 1M divider , 5,2V peak to peak could be correct
So at the grid there is much less signal than roughly half that would be from 1M + 1M divider , 5,2V peak to peak could be correct
Nope, no NFB.
As the cathode signal is in-phase with the grid signal, it's actually positive feedback with a gain of about 0.5.
Result is bootstrapping of the 1M resistor, thus increasing the input impedance to around 2M.
As the cathode signal is in-phase with the grid signal, it's actually positive feedback with a gain of about 0.5.
Result is bootstrapping of the 1M resistor, thus increasing the input impedance to around 2M.
5.3Vrms at the grid would mean over 130Vrms at the plate. That's not possible without heavy clipping.
The previous stages gains are ok , the signal should be a lot higher without lossy tone corrector and another voltage divider at the reverb
And the volume pot is at half
And the volume pot is at half