Budget Express - Traditional Build Expectations

[RJ Guitars]

As most of you know I seldom hold to the traditional ways of building the Trainwreck Circuits but in this case I offered to finish an amp for a friend whose eyesight was failing him. Stack O' Caps and Copper Buss Bar soldered to the back of the pots all included.

I put an attenuator on it and played it hard for a while tonight and it sounded really good. When I was done I was surprised at how hot that 1K 25W resistor was. I have typically avoided this resistor in this location and used a chassis mount that I can heat sink... but in this case I was going for the traditional method. How hot should I expect that thing to get?

I set the voltage on pin 5 at -30V... it's probably not the ideal spot but should have me in the "safe" zone I think. I took a quick look at voltages and overall everything looks pretty good. The transformers are way cool and the only other hot items were the 1K 5W resistors on the power tubes... but they didn't seem as hot as the big 1K. I will probably add the 1 ohm resistors just to verify where I really am at on each tube.

Just want to make sure that I am not missing something. It seems to be working well but I want to make sure I have the fundamentals right before I set off to tweak it.

rj

[badtweed]

Maybe check for ultra sonic oscillation with a scope to eliminate that possibility.

[Markusv]

When I was done I was surprised at how hot that 1K 25W resistor was.

rj

I looked at some of the Francesca gut pics recently and was surprised at how seemingly close the 1K 25W resistor was to the wires that run underneath it. I have felt some heat on that resistor in my builds and would never do that. But then I'm not Ken Fisher and he must have thought about it.

Markus

[RJ Guitars]

Maybe check for ultra sonic oscillation with a scope to eliminate that possibility.

Interesting thought - I'd be curious to learn if you seen something like that before? What would generate the oscillations?

[RevD]

I'd guess Ken must have been expecting some heat there to make it a 25 watter? Saying as a person who hasn't built one, but its awefully large by any amp I've ever seen as far as a power resistor is concerned?

Regards,

Don

[martin manning]

Maybe check for ultra sonic oscillation with a scope to eliminate that possibility.

Interesting thought - I'd be curious to learn if you seen something like that before? What would generate the oscillations?

You could have an oscillation going in the output section due to say some lead dress problem, and the power stage would be drawing high current with no audible sound coming from the speaker. If the voltages along the power supply string and the quiescent bias current look reasonable, and the amp's volume seems normal, then you can probably rule that out. As far as I can tell that resistor should only be dissipating ~2.5W running flat-out. I think power resistors run at higher temperatures than most people are aware.

[jelle]

How do you know?! We do not know the bias current.... Do the screens light up a lot?

[rooster]

Glen has thoughts about this piece in the direction of hot but I do not. I can play the amp hard and it will get warmer, sure, but never to where I couldn't put my finger on it and keep it there.

Sometimes when this comes up I think about the fact that Ken has 80uf in front of it. If the caps are doing their job well, I think the 1K resistor is not working hard. On the other hand, if the amp is gating/pumping because it is voltage starved then this piece is probably working a lot harder. ....And here I am thinking about the JTM45 circuit with 32uf in front of the choke. The choke in this case is a really massive tranny as it should be.

So check your caps. Maybe they are not formed.

[RJ Guitars]

Yes, I want to make sure everybody is thinking about the big resistor in the power supply... then it's also important to note that I mentioned I put this on an attenuator and played it hard for a while. It was believable that it got hot, but I was surprised at how hot I was able to get it. In a half hour it was pretty hot - not too hot to touch but getting close. Interesting, but maybe not a surprise if I went through all the math. It just wasn't intuitive to me that I would be able to warm up a 25 watt resistor and still the transformers stayed cool to the touch. I figured if it was working that hard everything would be warm.

I'm inclined to go on with the tweaking but I will throw the scope on the nodes just out of curiosity at this point.

rj

[jelle]

What is the bias current at idle in mA per powertube?

[RJ Guitars]

What is the bias current at idle in mA per powertube?

I had better chance to open up the amp today and listen a little more critically and it seems like the tone was more compressed than I cared for so I just now changed the bias current but up until now it was ~38mA. I moved it to 45mA and I'll give that a better listen tomorrow and see if I like that better.

Here are the voltages on it right now:

409V - B+ node 1
390V - B+ node 2
308V - B+ node 3
289V - B+ node 4
272V - B+ node 5

165V - V1a plate & 1.57V - V1a cathode
200V - V1b plate & 1.90V - V1b cathode
255V - V2a plate & 3.00V - V2a cathode

177V - V3a plate
212V - V3b plate

409V - V4 Plate & 386V - V4 Screen
409V - V5 Plate & 386V - V5 Screen

I have looked at a lot of other folks voltage data and I don't typically see the big disparity in voltage on the Phase Inverter that I have on mine. I have not done any tweaking yet except to run the bias up tonight. So far no tube swapping either. Do folks tend to adjust the PI plate resistors to balance these? I used 82K and 100K - maybe I better look and see if I accidentally swapped them??

Does anybody see anything else that seems way out of spec?

rj

[martin manning]

Those voltages look reasonable except for the disparity at the PI plates. If the plate resistor values are correct that has to be a case of poorly matched triodes. Have you tried a different tube?

[jelle]

Rj, thanks for the data. +1 on what Martin said.

The hot resistor could still be excessive screen current. The idle numbers appear ok at first sight. What is the voltage drop of the screen resistors both at idle and at full tilt?

[geetarpicker]

I've noticed the 1k 25 watt resistor gets pretty hot in my original Express after playing the amp into full saturation for a while. However note that the wires running underneath it are bent in such a way as they stay flat against the board and do not touch the bottom of the resistor plus there is a small air gap in between the wires and the resistor. With over 2+ decades of use there seems to be no sign of heat damage to the board or wires.

[eazilyled]

RJ, I was planning to swap out the EL34s in my clone last night, so I was able to measure the temperature of the resistors in my clone using a fairly cheap point and click infrared thermometer with laser target.

First I measured the temp with the original tubes which were Winged C biased at approx 45ma idle cathode current. When I originally biased these I determined that one of them drew a fair bit more plate current AND screen current than the other (plate current difference was approx 4ma and screen current difference was approx 2ma). So really one tube was biased at 45mA and the other at 39mA idle. This difference in draw followed the tubes rather than the socket, so I knew that I had some slightly mismatched tubes rather than the imbalance being caused by the amp itself.

After 5 mins of heavy playing (volume at 1 o clock), the temperature of the big 1K resistor R25 was approx 35C, and the screen grid resistors (R20 / R21) were in the mid to late 30s (C). Maybe the proximity of the hot EL34s contributed to the high temperature of the screen grid resistors, I don't know.

I then dropped in some RFT EL34s and biased them to approx 43mA cathode current (I wanted to bias a bit more conservatively on these). The plate and screen current on these were quite similar (within 1mA).

After 5 mins of heavy playing at the same volume, this time the temperature of R25 was approx 25C, and R20 / R21 were largely the same as before.

Really I just wanted to get a feel for what was a 'normal' temperature, at least normal in my amp. But it was interesting seeing this difference in temperature for R25 between the two sets of tubes, even though I can't say what caused it. Jelle's comment about the temperature of the resistors being related to screen grid current certainly makes sense given the position of R25, R20 and R21 in the circuit.

I know that this was far from a controlled experiment, as the bias current wasn't the same in both cases, and I can't guarantee that I used the exact same volume setting or played hard for exactly same period of time. It was even tricky getting a steady reading with the thermometer.

But hopefully that gives you something to compare against.

Also, in both cases the transformers were cool to the touch as you found. The only time I've got the power transformer warm (but still comfortable enough to rest my hand on it) was after blasting an iPod through the amp for a few hours when trying to break in the Sozo caps in the amp.