I think the guy wanted to avoid the PI supply voltage dropping to below 300V with the 10k.
This makes sense especially as the 807s need more grid drive than EL34s.
I'd leave the 1k alone but add a 10k dropper between the parallel 50µ filter caps.
I dont think 10K would do that going by what the voltages are reading further down with 10K added.
I will leave the 1K in plase as you say, but I might need to increase the others to around 20K or so to get the desired voltage.
They are still reading in the mid 300's with 10K (see post #38)
20k won't bring you there, as there will be few tens of V less than with 10k.
I'd suggest you to try both and hear the difference: 10k instead of 1k for the PI, and 20k instead of 10k for V1 and V2.
Otherwise you can keep the voltages as they are, and convert the amp to a Soldano or VHT circuit, that are more towards the voltages you have now.
I'd suggest you to try both and hear the difference: 10k instead of 1k for the PI, and 20k instead of 10k for V1 and V2.
Otherwise you can keep the voltages as they are, and convert the amp to a Soldano or VHT circuit, that are more towards the voltages you have now.
First off, why would the volume positions or load on the amp matter to the steady state condition at idle? What would make the voltage drop 20-25V?
411V > 1K >400V > 4K7 > 378V > 4K7 > 372V
411 - 400 = 11V. 11 / 1,000 = 11 mA
400 - 378 = 22V 22 / 4,700 = 4.7 mA
378 - 372 = 6V 6 / 4700 = 1.3 mA
----------------------
With a 10k resistor.
411V >10K > 360V > 10K > 348V
411 - 360 = 51V. 51 / 10,000 = 5.1 mA
360 - 348 = 12V 12 / 10,000 = 1.2 mA
Missing a stage's measurement? Let us pretend the first stage wants 1 mA and you have a 10k in place.
10,000 x 1.0 mA = 10V
Now let us compare it to Marshall's figures. Might as well use the 100W MV amp since you want higher voltages.
470V > 10k > 330V > 10k > 290V > 10k > 280V
470 - 330 = 140V 140 / 10,000 = 14 mA
330 - 290 = 40V 40 / 10,000 = 4 mA
290 - 280 = 10V 10 / 10,000 = 1 mA
So where does it look like the "problem" is?
I naturally thought that extra load would bring the voltages down a bit, depending on how the power supply can handle the load.
It is a 100+W amp after all, but nonetheless the anode voltages on these preamp tubes are a bit on the high side for my liking.
I will try swapping out the 1K resistor for a 10K for a start i think, the marshall amp originally had a 380V supply voltage in this position from what I can gather, so if im working with a higher 400V supply and yet using a much lower resistor in this place, that is likely the problem?
It is a 100+W amp after all, but nonetheless the anode voltages on these preamp tubes are a bit on the high side for my liking.
I will try swapping out the 1K resistor for a 10K for a start i think, the marshall amp originally had a 380V supply voltage in this position from what I can gather, so if im working with a higher 400V supply and yet using a much lower resistor in this place, that is likely the problem?
OK so finally got round to doing some load tests and the voltage changes a fair bit on full volume.First off, why would the volume positions or load on the amp matter to the steady state condition at idle? What would make the voltage drop 20-25V?
411V > 1K >400V > 4K7 > 378V > 4K7 > 372V
411 - 400 = 11V. 11 / 1,000 = 11 mA
400 - 378 = 22V 22 / 4,700 = 4.7 mA
378 - 372 = 6V 6 / 4700 = 1.3 mA
----------------------
With a 10k resistor.
411V >10K > 360V > 10K > 348V
411 - 360 = 51V. 51 / 10,000 = 5.1 mA
360 - 348 = 12V 12 / 10,000 = 1.2 mA
Missing a stage's measurement? Let us pretend the first stage wants 1 mA and you have a 10k in place.
10,000 x 1.0 mA = 10V
Now let us compare it to Marshall's figures. Might as well use the 100W MV amp since you want higher voltages.
470V > 10k > 330V > 10k > 290V > 10k > 280V
470 - 330 = 140V 140 / 10,000 = 14 mA
330 - 290 = 40V 40 / 10,000 = 4 mA
290 - 280 = 10V 10 / 10,000 = 1 mA
So where does it look like the "problem" is?
Most valve stages are dropping at least 10-20V or more meanwhile there is actually a voltage increase on V2A.
So as it stands at idle, the voltages are 411V > 1K> 400V > 10K> 355V> 10K> 345V
Now under full load they are 380V> 1K> 372V> 10K> 332V> 10K> 320V
I then swapped out the 1k for a 10K and I get 411V > 10K> 339V> 10K> 301V> 10K> 291V
The tube voltages appear to be smack on at the anodes on the schematic, V2A is about 155V, not too sure what it should be because the schematic is hard to read.
Both the anodes on the phase inverter are close to 250V each.
Looks like dropping the 1K for a 10K sorted it.
The Marshall had 20K in this place, so 10K looks to be close enough with the lower voltage supply I'm working with here.
Only concern is this voltage drop while under load, is this typical, or is my power transformer inadequate?
Well it's not typical use to run the thing at full overdrive volume either but good stress test if nothing else.
I should be able to confirm this on my scope soon, once I replace all these parts I will do another test.
I don't know how guitar amps power outputs are typically rated at, given that they are kind of designed to have some level of distortion to give them their unique sound.
I don't know how guitar amps power outputs are typically rated at, given that they are kind of designed to have some level of distortion to give them their unique sound.
It's been so long and I'm trying to dig up my notes I had for this but iirc, it was 2K on the primary.
All amps are rated at max. clean sine output (just before clipping) into rated load.
Preamp distortion increases with signal level, so with a high gain amp the input signal must be very low (e.g. a few mVs).
With a sine wave just visibly clipping the distortion is suppose to be in the 5% range, generally where power measurements are done.
Guitarland is generally very open to interpretation, when we talk about declared max power.
Consider how the impedance can vary in a guitar speaker, and you'll see how different the output power can be on a sweep of frequencies.
Consider how the impedance can vary in a guitar speaker, and you'll see how different the output power can be on a sweep of frequencies.
looks at Marshall are you sure on that? It seemed to be typically taking the tube power output for 2xel34 or 4xel34 as 50/100W and they're not running clean at that power output.
With the cold clipper and cold front end of the JCM, it's definitely not going to be scope 'clean' at any normal volume. A JTM45 is a different frontend (coupled cathode instead of cascade and no cold clipper) and so cleaner.
I found the high front end voltages on the JCM gave a laster cleaner/harder sound, the lower front end gives more of a legacy sound. Both good for their respective options.
To measure power amp output capability I feed my generator signal to the PI input, thus avoiding preamp effects.
I would skip the max power-rating parameters chosen by instrument amplification builders, because they are 99% of the times commercial declarations and not technical ones: buyers need to know if it's 30, 50 or 100. Then they don't really care how much is it. NFB controls on the power amp (feedback, depth, presence) make the result frequency dependant, so even less sense to measure it except as a reference range.
Voltages I suggested are in the vintage-sounding range. I personally prefer higher voltages for higher and more modern gain sounds.