36Watt Weber Rebuild Problem - Please help troubleshoot :)

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treatmentroom
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36Watt Weber Rebuild Problem - Please help troubleshoot :)

Post by treatmentroom »

I bought a used 36watt that ended up being a Weber kit that had been modded in some questionable ways. I decided to gut most of it and rebuild with better caps using this layout:
http://taweber.powweb.com/store/6m36tmbh_layout.jpg

Both channels are making sound and the eq's seem to work as they should at very low volumes but when I dig in or turn the volume up (even just to 1), both channels make a TERRIBLE overloaded sound.

Here are the voltages I'm getting.... they seem a little high based on the voltage chart in the download section but i'm not sure if they are within the window of acceptability :):
V1:
1. 174v
2. 0
3. 1.34v
6. 173v
7. 0
8. 1.35v

V2:
1. 236v
2. 59v
3. 86.5v
6. 249.6v
7. 62.3v
8. 86.6v

V3:
1. 162v
2. 0
3. 1.28v
6. 265v
7. 161.4v
8. 161.6v

el84s
2. .75v
3. 10.97v
7. 376v
9. 368.8v

Please let me know if you need more info or have any advice. Thanks so much!
Last edited by treatmentroom on Tue 11/20/12 8:44 pm, edited 1 time in total.
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Post by treatmentroom »

I will add that the amp has a WZ34 Copper Can rectifier and the B+ at the first cap is 384vdc which from my searches on here seems high..... yes? Could that be the problem or is it probably further down the line?

Thanks a ton for all the info shared on this site and I appreciate any help. I will try to post pictures of the amp in a while.
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Post by zaphod_phil »

Yes, that voltage is way too high (assuming you're measuring with tubes plugged in and running).
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Post by treatmentroom »

Thank you for the response..... I thought it might be. Yeah that is with tubes in and running.

I'm pretty new to this type of trouble shooting so I'm definitely on the upward part of the learning curve. Would you suggest that I raise the first 5w resistor on the first filter cap to a 1K like on the 36watt in the download section? Right now it's a 470.

If not that, what would you suggest?

Thanks.
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Post by treatmentroom »

I tried a 1k-5W and it brought down most of the voltages by 8-10 volts and the amp seems a good bit happier.... it's no longer screaming in pain :)

According to the voltage chart I'm in pretty good shape on the PA and PI voltages but mainly need to bring down the grid and plate voltages on the power tubes by 20-30v but I'm just not sure of the best way to do it.

Is using a zener diode on the B+ my best option?

Any insight/wisdom would be amazing.

The current voltages are:
V1:
1. 165.6v
2. 0
3. 1.32v
6. 164.9v
7. 0
8. 1.32v

V2:
1. 227v
2. 58.2v
3. 84v
6. 238.8v
7. 60.3v
8. 84v

V3:
1. 154.2v
2. 0
3. 1.26v
6. 254.7v
7. 154.4v
8. 154.6v

el84s
2. .07v
3. 10.48v
7. 373.7v
9. 355v
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Post by rock_mumbles »

You will need to use a string of 5 watt zener diodes to drop about 30 volts! Four 7V 5 watt zeners should do it ... keep the diode leads kind of long to help dissipate the heat ...

The other thing you could do if you don't mind getting a bit of sag like a tube rectifier would be to put a 100 ohm 10 watt resistor between the rectifier and the first filter cap ... that would drop about 15-20volts ... then use some zeners in addition to get the voltages to where you like the amp. Zener diodes are not resistive so the voltage drop doesn't change with current demands like a resistor.

The 50 ohm 10 watt screen resistor may not limit the screen current enough for new tubes ... pushed hard the plate voltage may drop below the screen voltage which doesn't sound good and could cause the screens to burn up. I put 1k 3 watt screen resistors on each tube. You could use a single 250 ohm resistor for that I'd guess that a 10 watt resistor would be good enough.

Once you lower the EL84 voltages you may want to put the 470 ohm dropping resistor back in the power supply. I'd play it with the 1k resistor first, if the amp sounds too dark or breaks up too easy then replace the 1k with the 470 ohm.
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Post by treatmentroom »

I may try the 100 ohm 10 watter and see what happens.

What about switching to a tube rectifier? The amp came with the Copper Can but I'm not married to keeping it. Would switching to a tube rectifier be less efficient and drop the voltage enough?

Being a Weber kit I would have expected the voltages to be closer. All i did was switch the coupling caps for Sozos.... any thoughts on why the B+ would be so hot?

Thanks again for all the help guys ... I'm learning a lot.
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Post by treatmentroom »

Progress :)

Thanks for the tips Rock. I'll switch that screen resistor. Anything else in that layout look out of place to you?

I pulled an 5AR4 from my Deluxe Reverb and now things are a bit more in line. I switched the resistor on the first filter cap back to 470 ohms. NOW i'm a little low on voltage downstream.... The amp is sounding better but the TMB channel sounds overly saturated to me and is too fizzy even at 2-3 on the volume. The normal channel sounds better but the tone control doesn't seem to make much change until it's below 4. I noticed that in Richie's layout the tone control is wired a little differently than the Weber so maybe I will try changing it.

Current Voltages:
B+: 340

V1:
1. 152.3v
2. 0
3. 1.2v
6. 151v
7. 0
8. 1.2v

V2:
1. 207.8v
2. 53.1v
3. 76.8v
6. 217.4v
7. 54.8v
8. 76.8v

V3:
1. 130.3v
2. 0
3. .94v
6. 231.2v
7. 130.4v
8. 130.4v

el84s
3. 9.26v
7. 328.2v
9. 321.3v

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Post by rock_mumbles »

With the 5AR4 your plate voltages are now about where I like them in an 18 Watt ... I'm surprised that it dropped the voltages that much as a 5AR4 has the least voltage drop of common tube rectifiers.

As far as your low preamp voltages that's because of the 10k preamp dropping resistor, once you get the EL84 voltages where you want them, either with a sag resistor and zeners or a 5AR4, then reduce the 10k dropping resistor until you get the preamp voltages up where they should be.

As far as the 'normal' channel tone control, the tone control used on the LiteIIb is generally considered a better tone control (it's not as dark)

I just looked at the circuit in detail ... it's quite different than other TMB circuits I've seen ... for one thing it's missing the master volume (16mm pot) usually installed in one of the input jack holes ...

The normal stage is pretty normal with a 100k plate resistor and 1.5k||50uf cathode ... if you want more gain in the normal channel change the plate to a 200k.

The amp is set up more like an 18watt TMB than a 36watt ... the TMB channel has no interstage attenuation ??? that could be responsible for not being able to get the volume control over '3' ...

The input stage triode has a 100k plate resistor with non-bypassed 1.5k cathode to a 0.0047uf coupling cap directly to the 500k gain (volume) control.
The next gain stage triode has a 100k plate with 1.5k||50uf cathode directly coupled to a cathode follower which is followed by the TMB tone control to 0.022uf coupling cap into the PI.

Thinking "out loud"
Both gain stages are center biased (100k plate w/ 1.5k cathode) usually considered to be not a good setup for an overdriven tone. The other 36Watt TMB designs I've just looked at are set up more like a Plexi with two input stages that use mixing resistors into the second gain stage ... the first stage has the cathode bypass the second does not.

If it was my amp ...
I'd change the input stage to be cold biased with a 2.7k resistor and 0.68uf cap like a Plexi bright channel. That will make the tone crisper by reducing the lows and give more gain to mids and highs, it also takes pedals well. Between the 0.047uf coupling cap and the 500k gain (volume) pot I'd add a 470k resistor, this will reduce the signal into the next stage a bit. (You can add a bright, normal, thick switch across the 470k resistor)

If you do this mod stop at this point and play the amp, better worse ???

Then step two I'd change the second gain stage to a warm biased stage for creamy Marshally distortion. Put in a 820 ohm cathode resistor with a 1uf cathode bypass cap.

Be careful with what tube you put in V3, some tubes don't like the high heater to cathode voltage in the cathode follower. China tubes work well there.
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