Thumping from amp on startup

[JazzGuitarGimp]

I'm going to rebuild the power filtering layout on this amp.

I was going to use the 5E3 power filtering schematic as the template:

http://taweber.powweb.com/store/5e3_schem.jpg

Does this sound like a good plan?

Also, on the 5E3 schematic, I don't see the NFB. Does it not have any?

No, the 5E3 does not employ feedback.

[Firestorm]

I'm going to rebuild the power filtering layout on this amp.

I was going to use the 5E3 power filtering schematic as the template:

http://taweber.powweb.com/store/5e3_schem.jpg

Does this sound like a good plan?

Also, on the 5E3 schematic, I don't see the NFB. Does it not have any?

No, the 5E3 does not employ feedback.

Gibson Explorer (I think) is the same amp, but w/ 10" speaker. It does use NFB. Sounds lame.

[Firestorm]

I'm going to rebuild the power filtering layout on this amp.

I was going to use the 5E3 power filtering schematic as the template:

http://taweber.powweb.com/store/5e3_schem.jpg

Does this sound like a good plan?

Also, on the 5E3 schematic, I don't see the NFB. Does it not have any?

No, the 5E3 does not employ feedback.

Gibson Explorer (I think) is the same amp, but w/ 10" speaker. It does use NFB. Sounds lame.

[bal704]

I've rebuilt the power supply and power amp to mimic the weber 5e3 schematic. Here's a modified version of that schematic with my preamp in it.

[IMG658]http://i755.photobucket.com/albums/xx19 ... _sch_2.jpg[/img]

I'm using a 750 ohm/22uf cap from pin 8 to ground on each power tube.

The thumping is gone, and the amp sounds pretty good. There's no major problems, but there is a vibrating sound on notes. The easiest way to describe it is it sounds like fret buzz coming through the amp (the guitar has no fret buzz). It's easy to hear on the clean channel, not as easy on the higher gain channel. I'm going to go through the amp with chop sticks to see if there's a bad connection or something.

Thanks for the help so far.

[Structo]

Where is your heater voltage sitting?

Could be filament rattle or microphonic tube.

[bal704]

It definitely has a 'rattle' feel to it. I'll have to investigate filament rattle, since I know nothing about it.

I'll check the heater voltage tonight and report back.

Where is your heater voltage sitting?

Could be filament rattle or microphonic tube.

[bal704]

Here's what my voltages look like, from the schematic above.

Voltage A = 370 VDC
Voltage B = 341 VDC
Voltage C = 275 VDC
Power tube 3-8 339 VDC
Power tube 8-gnd = 31 VDC
Heater = 6.92 VAC

Also, it appears the buzz I describe above has something to do with channel 3 on the amp. As I mentioned, there are 3 identical channels, running through weber channel switchers. The buzz only occurs when there is a tube in channel 3. Channel 1 and 2 sound fine as long as there is no tube in 3. If I have a tube in 1 and 3, I get the buzz.

I'll do a little tidying up and see how it sounds.

[Structo]

You don't really have 31v on pin 8 of the 6L6GC do you?

Watch the heater to cathode voltage on the CF.

[bal704]

I have 31 VDC from pin 8 of each 6L6GC to ground. I'm using a 750 ohm resistor for each cathode.

I have 339 VDC between pin 3 and pin 8. According to the weber bias calculator that should be about 40 mA and 13 watts per tube.

I'm not sure what you mean by the heater to cathode voltage on the CF.

You don't really have 31v on pin 8 of the 6L6GC do you?

Watch the heater to cathode voltage on the CF.

[Structo]

Yeah I come up with 41ma and 14 watts per power tube not counting screen current.

Forget what I said about the heater/ cathode voltage.
At first glance I didn't realize you were using a cathodyne inverter similar to the 5E3 Deluxe.
Here is an excellent article on that inverter.

http://www.freewebs.com/valvewizard/cathodyne.html

You might experiment with different tubes in that inverter spot.
A 12AU7 while lower gain can provide a greater swing although it may not like real high plate voltage.
For greatest gain stick with the 12AX7.


The heater voltage at 6.92 is a bit high.
Your tubes will last much longer if the heater voltage is at the spec 6.3vac.
A single silicon diode of the proper rating in line with the heater string would drop about .7v