Weber 5E3x2 heat issues

[Chris G]

Just built a Weber 5E3x2 kit from Weber.
It sounds great, but the chassis is a little on the warm side.
Well, hot actually.

The voltages are OK, and I've read where Weber has inserted a big ol' dropping resistor in the PS to keep the heat down, but this bad boy is still really, really hot.

I mean, I can't touch the chrome for more than 5 seconds.

Any ideas?

I'm using a copper cap rectifier, by the way.

[Bruce / Mission Amps]

Many tweed amp chassis can get hot just by the way the PT and tubes are mounted, especially with four power tubes and the Copper cap.
However, I'd get rid of the Copper Cap and use a real rectifier, a glass NOS 5R4 or NOS Svetlana 5U4G.
Also, if weber is still using the PT I helped R&D, I've always thought it was a little light weight for this amp, espeically if the power tubes are idling at +12 watts each. Cool those power tubes down and it should be OK.
I know the Weber PT is rated at 200ma but while testing those on my bench set up (during R&D days with weber), with a 200ma load on the PT, it got hotter then a $2 pistol in a back alley on Saturday night.
It did not fail but ran way hotter then I expected at full load. Much more like an overloaded 150ma PT to me.
So, you might want to rebias the power tubes a little cooler and make sure the PT is wired to the lower high voltage taps.
Oh, don't use "a big ol' dropping resistor" after a recitifer to control plate or B+ voltage. Its a waste of power supply energy, the voltage drop is all over the place depending on the current flow through it and creates a lot of heat in the resistor.
Use a few 9v 5 watt zener diodes in the center tap of the high voltage secondary to lower the actual B+ by a few volts.
With a real NOS 5R4 or NOS Svetlana 5U4G rectifier you won't need to reduce it much, if any, especially if you also make sure the power tube's cathodes are sitting on a 180 to 220 ohm 10 watt cathode biasing resistor with a 68uF toi 100uF 100 v cap across it. And those were some hints.

[Chris G]

you also make sure the power tube's cathodes are sitting on a 180 to 220 ohm 10 watt cathode biasing resistor with a 68uF toi 100uF 100 v cap across it. And those were some hints.

1. I've got a NOS 5U4GB lying around, I'll pop that in. Are you saying the 50V that tube rectifier will drop is MORE than what the copper cap drops?

2. The cathode arrangement is stock: a 150R/10W resistor w/ a 50uF/50V cap across it. I'll beef it up tonight to 200-250R with 100uf, thanks for that tip.

3. I just grokked the old-skool 5E3 schematic, which shows 250R and 25uF on the cathode. Is there a significant difference, sound-wise, with those values?

[Chris G]

Update: heat issue resolved

Here's what I did:

1. Subbed out the stock Weber cathode resistor/cap arrangement with 330R/100uF. Sounded bad. Went down to 180R/100uF.

2. Subbed out the WZ34 copper cap with a 5U4GB I had.

It still sounds amazing, and it's even a little cleaner on the low end (around 2-3) of the volume pots. The heat level is now normal. The tranny is running plenty cool now.

Thanks, Bruce!

[Bruce / Mission Amps]

Here's what I did:

1. Subbed out the stock Weber cathode resistor/cap arrangement with 330R/100uF. Sounded bad. Went down to 180R/100uF.

2. Subbed out the WZ34 copper cap with a 5U4GB I had.

It still sounds amazing, and it's even a little cleaner on the low end (around 2-3) of the volume pots. The heat level is now normal. The tranny is running plenty cool now.

Thanks, Bruce!

OK very good Chris...

1. 180 ohms is my choice for the four 6V6s but I have gone up to 220 with good results when the B+ was a bit higher...
if you have two of those 330 ohm resistors, put one on each opposite side of the power tubes with their own cathode bypass cap so you can yank two inside or outside tubes out of their sockets and still have a balanced push pull amp with decent bias on two power tubes and half power.

2. A NOS 5R4 is my choice here but a NOS 5U4G, 5U4GB is still OK and better then a WZ34 or GZ34. If you want a little more vintage look and tone, the NOS Svetlana 5U4G (ST, coke bottle looking tube) is a good choice.

3. After a few hours of playing with it, its time for some tone tweaking
********************
" I just grokked the old-skool 5E3 schematic, which shows 250R and 25uF on the cathode. Is there a significant difference, sound-wise, with those values"
250 ohms (on up) to cathode bias four power tubes, (especially at the slightly lower B+ voltage using the 5R4 or 5U4G) will have the power tubes a bit over biased and they will sound a little cold, sometimes harsh/brittle.
When using smaller resistance value cathode biasing resistors, up the capacitor value... 250 ohms and 25uF in minimum in my book while 180 ohms with a 68uF to 100uF is good.

[Chris G]

if you have two of those 330 ohm resistors, put one on each opposite side of the power tubes with their own cathode bypass cap so you can yank two inside or outside tubes out of their sockets and still have a balanced push pull amp with decent bias on two power tubes and half power.

Hokay ... that sounds great. That would let me keep playing if a tube died mid-set. But these (admittedly great) suggestions raise more questions for me:

1. As for the caps: Do I keep the values the same? Would running all four tubes with two caps act as paralleling the cap values? Should I install 90uF (likely 100uF, for availability reasons) on each side? Or 180uF on each side, like I have now?

2. As for the resistors: Will the crappy, super-brittle sound I got with 330 ohms on four tubes be less crappy if I pull two tubes? In other words, with one 330R resistor on each side, the cathode resistor part of the circuit would look like 165R to FOUR tubes, yes? Would it still look like 165 ohms to two tubes?

3. Also, speaking of tweaks: Is it possible to wire either an inside or outside pair of sockets for a 6L6GC, keeping the other pair wired for 6V6? (Use separate B+ and bias supply from the unused 360V and 55V PT wires, etc.) Would this be a good thing? Is this an idiotic idea? I popped a couple of RCA blackplates in there alongside some 6V6s without changing anything else and didn't notice much difference. The OPT has a 4K primary and is spec'd for 40W.

[Jeff E]

2. As for the resistors: Will the crappy, super-brittle sound I got with 330 ohms on four tubes be less crappy if I pull two tubes? In other words, with one 330R resistor on each side, the cathode resistor part of the circuit would look like 165R to FOUR tubes, yes? Would it still look like 165 ohms to two tubes?

You can answer your own question about the crappy sound by pulling a pair of tubes. (psst... yes)
What Bruce is suggesting is connecting the cathodes of the inner pair of tubes to one 330 ohm resistor/bypass cap, and doing the same with the outer pair to another resistor/cap. The two circuits are not connected and operate independently, but with all tubes in it is the equivalent of running the stock circuit with 165 ohms.
You could also wire a switch to accomplish pretty much the same thing.
I used a 3PDT from Mouser to select the appropriate output impedance for each setting.

[LarryLarry]

I'm confused. The Hi/Lo switch diagram shows the cathodes of V3/V5 together and V4/V6 together. Is Bruce saying to connect the cathodes of V3/V6 together and V3/V4 together? Does this matter? I see the plates of V3/V4 are connected to one side of the OT and the plates of V5/V6 are connected to the other side of the OT.

Can someone clarify? For now, I don't want to do the switch, I want to do the dual 330R/100uF thing and just pull 2 tubes for half power...

[hasserl]

V3 and V4 are on the same side of the OT, and V5 and V6 are on the same side of the OT, so you can disconnect V3 and V5 or V3 and V6; Or you can disconnect V4 and V5 or V4 and V6, it really doesn't matter. All that matters is that you disconnect (or remove) one tube from each side of the OT. This way you will still have two tubes (one per side ot the OT) still connected.

What does matter is that you decide ahead of time which tubes you want to do this with, and wire it up accordingly. If you connect 3 & 5 together and 4 & 6 together, then when you pull two tubes you'll pull one of those pairs, not one from each pair.

Hope that makes sense,
Hasse

[LarryLarry]

Ok, so I connected the cathodes of v3/v6 and v4/v5 and ran a 330 ohm 10 watt resistor bypassed with a 100uF/100v capacitor to each pair. It's awesome! I pulled the 2 inner tubes and then the 2 outer tubes and it works great at 1/2 power! Not only that, but I think it cleaned up a little bit of muddyness

Thanks Bruce et al...

[Bruce / Mission Amps]

Glad you got it figured out as this little mod makes the amp a little more versatile too.

[LarryLarry]

When I pull 2 tubes, can I use 2 5881s or 6L6s instead of the 6V6s? I realize this kind of defeats the purpose power-wise but I would like to give it a shot tone wise.

[Bruce / Mission Amps]

When I pull 2 tubes, can I use 2 5881s or 6L6s instead of the 6V6s? I realize this kind of defeats the purpose power-wise but I would like to give it a shot tone wise.

The OT primary zed for two 6L6s/5881s is about the same as the four 6V6s.
As lone as the cathode biasing resistor is a power resistor that can handle the amount of current needed to run two 6L6 or 5881s, I'd say yes.

[LarryLarry]

Ok, I gave this a shot. Just for reference, there is not too much of a volume difference when pulling 2 of the 6V6s, but there definitely is some for sure. A definite loss of some headroom with 2 tubes, although headroom isn't why we play these amps now, is it Enough difference though to accomodate playing a quiet/small room if needed.

My 6V6s are idling at 34mA measured with a bias probe (one at a time) with 360 volts on the plates and 22 volts on the cathodes. They stay consistent with either 2 or 4 tubes installed. I tried a pair of Tung Sol 5881s and the were sitting at 46mA and I tried a pair of old Sylvania 6L6s and they were at 48mA. I also tried a pair of GT KT66HPs and they were about 43mA. All of these sounded not much louder than just a pair of 6V6s and actually I thought they were a little bit mushier. Maybe I should change the impedance switch from 8 ohms (1x12 8 ohm speaker) to 4 ohms?

[hasserl]

Maybe I should change the impedance switch from 8 ohms (1x12 8 ohm speaker) to 4 ohms?

When running the 5881's or 6L6's, no don't change the impedance. If running only two 6V6's, and if your amp has an impedance switch, than yes you could set the switch to the 4 ohm setting. That will give you the correct primary load for two 6V6's.