Budget Express - Traditional Build Expectations

[jelle]

That was a great test, and clever too. You just measured the temperature increase due to the energy flowing through the resistor, which is a function of both DC and AC currents. Now, given the fact that your sampling n is only 1, we have to be careful to draw quick conclusions here but this is what the data suggests to me.

The temperature rise of the 25w resistor is decreased with 10C when using the RFTs. So with signal applied, the RFTs draw less screen DC current. We know that the AC part is shunted via the screens filter cap. This filter cap shunts the screen AC currents going through the 5W screen resistors to ground. These resistors are at the same temperature as before but the DC current was less than with the winged C tubes. As the temperature was the same, the AC component had to be increased to compensate.

So with a n of 1, it appears that the RFTs draw significantly higher AC currents under load. I know from experience that the screens in fact light up a lot under load.

Also, if the winged C does not behave like the RFT in this respect, which current production tube does? This might provide sn answer to RJ's question.

[RJ Guitars]

So I have had a chance to work on this and play a little more through the amp while I listen to it and make some measurements and a few changes.

I was testing it out with a Strat with stock pickups and also a Carvin with a set of hot humbuckers (13K) Either way there is not much headroom. It makes some great sounding crunch but it will need more headroom to have any utility to it.

I added the 1 Ohm resistors and set the bias at ~45mA.

I fixed the disparity between the two sides of the phase inverter - a bad EI tube. ~200V per side now.

I do think that the amount of heat I am seeing on that big resistor and the lack of headroom is telling me about some sort of problem(s) when I start cranking this amp.

Using my multimeter I can see as much as 60 or 80 volts drop across that power resistor when I wang on the guitar. That seems like a huge power supply sag to me but I've never had any reason to sort this out before so I can't say if it's normal or if it's even legitimate to measure it with a multimeter. I put the scope on it but all I could see was some very low level noise and nothing that looked like an oscillation. I plan to play with the scope more tomorrow and see if I can get a better look at that power supply sag.

I may lift the PS board and cut off the other set of power supply secondaries in case they are part of a lead dress issue. They are coiled up under the board.

My prime suspect is the PS filter caps, they are part of the low budget parts in this build but I'm curious what other ideas are out there.

rj

[eazilyled]

Thanks Jelle, for the detailed explanation of what could be going on with the temperature difference. I think it's easy to get trapped and only think of voltages in tube amps. It's refreshing and informative to think about when and where current flows.

RJ, I wonder if that sag is normal? It would be interesting if you could measure the temperature of that resistor somehow, e.g. does your multimeter have a temperature function for use with a thermocouple?

On the headroom front, is it that it doesn't clean up well with the guitar volume or is there no clean range with the amp volume knob?

On mine, it starts to get dirty pretty quickly on the amp volume. Maybe below 9 o clock is where clean lives. Also I was a bit suprised about how far I had to dial down my guitar volume to get a clean sound (335 copy with humbuckers) - I have to go down to say 1 to do this and then its quite twitchy. That said, the volume difference between clean and dirty is minimal, so I put this down to the taper of the pots in the guitar. I have cheap CGE pots in there at the moment, and I am planning on swapping them out with the Gibson Historic pots that have been discussed in another thread to get better control of the cleans.

[RJ Guitars]

That sounds very similar to mine on the clean headroom. Volume on the guitar less than 4 and amp volume 3 or less. I suppose this is due to putting the power tubes into distortion so early. I'd be incline to let all this happen a little later. Maybe more time to test this week end.

[eazilyled]

Yep, they're gainy beasts for sure but that's the way I like it

Maybe a lower gain tube in V1 like a 12AY7 or even a 12AU7 will get the amp into the desired zone. I know that Glen put lower gain tubes in V1 and V2 on his CD track 'Country Ham', and that's pretty clean. I wouldn't mind trying that myself.