Webb Amp Question

David Ball · From: North Carolina High Country

I got my Webb amp back from getting the Ken Fox treatment and it sounded great, but a few weeks later some new noises cropped up. Not surprising in an old amp, and especially not surprising since I've been playing through it so much since Ken did his magic. The noise is a hiss with a low level hum--I've had other solid state amps that sounded like this new, but this amp was very quiet before. The noise is in the power amp section--I can take the preamp out of the Webb into an external power amp and it's really clean and nice. (it actually sounds fantastic feeding a 5E3 Deluxe power amp stage).

So my question is this: are there any components in the Webb power amp that are known to need replacement when they get old? This is a pretty old Webb. I'm assuming I probably have one or more power transistors showing their age, but figured I'd ask if anyone had some words of wisdom before I drag out the o'scope and sine wave generator. Most of my amp experience is with tubes...

I will say that the Webb after Ken's treatment is one of the best sounding amps I've ever heard for steel. Very sweet indeed!

Thanks,
Dave

Tom Bradshaw · From: Walnut Creek, California, USA

Hi David. I believe I sent you this information back in June (if you are the same "David Ball"). The following information should have been included in the Schematic package I sent you. This information is designed for use by a qualified amp repair person. ...Tom

Test Procedure for AC Hum & "white noise"
(All Webb Amp Models)
Power Amp:
1. Use an 8-ohm speaker.
2. No input signal needed for the test!
3. Plug a shorted ¼” phone plug into the Effects Return jack (“Power Amp In”) on back of chassis. This disables the pre-amp.
4. Hook up DMM (Digital Multi-Meter) which must be of good quality.
5. Set DMM to low AC reading.
6. The Power amp output reading should be less than +/- 35 MV DC.
7. Power amp: B+ = +50; B-
8. B+ & B- Ripple Test: Quiet Amp = B+ 35 MV or lower; B- 18 VM or lower. Note: All B+ and B- readings must be taken directly across the filter caps.

Pre Amp
9. Output should exhibit no AC hum or white noise.
10. Set volume control to zero; treble, midrange & bass set to 5; set reverb to off. Set 3-Position Tone Selector Switch to middle position. Note: for Galaxy & GP amp models, set boost switches to "off". On GP amp only, set voicing to normal.
11. Remove shorting plug from power amp jack.
12. No AC hum or at most, a very small amount of white noise should be heard when the volume is turned up (with no inputs in either input jack).
13. When input cords are plugged into the amp, the amp should be fairly quiet.
14. With no inputs plugged in, set the volume and tone controls to maximum. Total noise voltage at output should be 5 MV or less. The average AC hum should be under 8 MV RMS. Any AC hum in excess of 16 MV RMS is unacceptable

Additional Clues in Noise Test: "What To Look For"”

1. Check all ground connections & power cord ground.
2. Check presence of metal shield on underside of cabinet (directly above the input jack area of faceplate). This metal shield must be present!
3. Check power amp filter caps for proper functioning.
4. Measure pre-amp power supply: 26.5 VDC +/- should be the reading across the electrolytics. 220 MFD on pre-amp board
5. With 8 – 4 ohm load on output and no signal input, heat sink should remain cool. However, a slight warmth build up is acceptable. If heat sink is hot with no signal, power amp is out of bias. Service power amp (get a schematic and start trouble shooting).

Another Problem: "When first turned on, the amp worked fine for about an hour, then goes dead."

One solution: By James Wood (Jwood36287@aol.com): "The amp was an early model (circa 1975). All parts were original. It had a positive and negative 26-volt circuit, unlike current models. It also had no Q15 transistor. I felt there was a faulty component in the EQ board. I concluded that some parts were old and were breaking down when a load was applied. Thus, the modern schematic did not show this. The Q14 transistor was still intact. I changed that transistor, the zener diode, the 10K resistor and the C-4 capacitor. I found that one of the three-pin connectors, when connected had 4 volts, when it should have had 26! The other pins had the proper voltages: 60 and 26. After I installed the new parts it returned to its proper voltage of 26. Everything then worked fine."

"I found this variation in a Webb amp, circa 1977: "On the modern schematic, C4 is listed as 470mfd @ 35 volts but the early model amp’s actual cap was a 220 @ 35 Volts. The schematic also shows the power transistor to be a T1P41C, but was actually a T1P41B."

PARTS & MOUSER NUMBERS:
CR13: 26-volt Zener Diode------------512-1N3750A
C4: 220mfd @ 35V Capacitor--------647-TVX1V221MAD
Q14: Power Transistor-----------------312-TIP41B
R40: 10k Resistor------------------------293-10K-RC

…James Wood

David Ball · From: North Carolina High Country

Thanks Tom--yeah that's me. Duh! I'm bad about reading the instructions. I was looking at the schematic, but ignoring the obvious...

Dave

Ken Fox · From: Nashville GA USA

The preamp out does not cut off the power amp on the older Webb amps, just a jack in parallel with the preamp output.
Did you disconnect the internal speaker and feed the preamp to the external amp to determine that the noise was in the power amp?

If so, there is a long tailed pair at the front of the power amp that could be the issue. As all the filter caps are new in the power amp section I would not suspect those. You can also disconnect the preamp signal via the cable from the EQ board to the power amp.

David Ball · From: North Carolina High Country

Yep--I disconnected the speaker. It doesn't have the sound of caps. More like what a bad plate resistor sounds like in a tube amp--hiss and crackle. Guess I need to dig out my scope.

Is it Q1 and Q2 that you're talking about here going into the power amp?

Thanks!
Dave

Ken Fox · From: Nashville GA USA

Yes. The older Webb schematic has difference from the newer schematic that Tom sent you. The older Webbs used 2N5323 or RCA1A10 there. The newer ones show ZTX795A

Des Hetherington · From: New Zealand

I have an old webb, and it works fine, but im wondering if it would be advisable to play Bass guitar through it at small gig venues? its a 614E combo with a JBL K130, cheers Des
_________________
Rains SD10 4 pedals 5 Levers, webb 614E,Goodrich Vol

Bill Cutright · From: Akron, OH

Des, I've done it. But, I'm not recommending it.
Even at perceived low volumes, bass guitar can really make that cone jump...

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