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JoelKnauber 13th August 2012 12:11 AM

Whitley PA Guitar Conversion
So, after gigging with my little VOX ad15vt for about a year, i decided i wanted a full tube amp. I'm a college student and cant afford anything too huge, but yet wanted something bigger than a kustom defender.

I grew an appreciation for PA to guitar conversions, and after looking on ebay for a while, i found one i thought i liked and bought it.

A WHITLEY "Murasonde" AP1000:

I chose this amp mainly because it uses 2x12AX7's, 2x6V6's, and has a 5Y3 rectifier, which I will replace with a SS rectifier to give the amp more bite.

Initial testing found the amp to be pretty crackly intermittently and it sounded like the tubes were pretty crappy. BUT it worked and have very little overall hum.

I started converting this amp by taking out the old power cord and installing a new 3 pronged cable, which is grounded to the chassis, and has an interrupter switch on the hot wire.

the power switch on this amp was located on the treble knob, and knowing that i needed to add a standby (SS rectifier) i relocated it to it's own switch on the side, along with a new standby switch.
(seen at the bottom of this picture)

Next i went to work on the inputs and outputs. (left side of pic)

The amp had 3 RCA inputs along with a selector on the front that you could select a LP filter and a lot of other unneeded things. So I found the outputs from a single channel on the switch and eliminated it entirely. Then wired in a 1/4" jack.
Next i wired 1/4" jacks for both the 4ohm and 8ohm outputs.

Put the chassis back together:

I tested this amp and was disappointed by finding that it didn't make any noise, and one of the ancient 6V6's was now arcing around in the bottom of it when i turned it up. I switched the tube in the socket and it did the same thing in the other socket, while the other 6V6 sat there fired up ready to rock and roll.

This is where I am still a rookie.. Did that tube finally just die after i took it out and moved it around? All I have done to it is what i have said above. Could something that I might have messed up cause this?

And help would be awesome!

kevinkr 13th August 2012 12:17 AM

:cop: Moved to Instruments And Amplifiers per forum policy when I approved the new thread. Please note the sub-headings under Tubes / Valves and Instruments And Amps.

kevinkr 13th August 2012 12:21 AM

Have you replaced the electrolytic and coupling capacitors in this thing? Until you do so you are playing Russian roulette..

Make sure that when no speaker is plugged in that there is some sort of load across the transformer secondary so that it is not fried inadvertently by being run hard while open circuit.

Tubes wear out, this one might have just reached the end of its life or there may be a tube killing problem in your amplifier. (bad coupling cap)

JoelKnauber 13th August 2012 12:26 AM

to be honest i'm not entirely sure what the coupling cap is.

is that the large 400ohm 7w one after the rectifier?

I haven't turned it up without the cab plugged in.

JoelKnauber 13th August 2012 12:39 AM

also, when i switched the tubes in the sockets, it didn't kill the second one.

kevinkr 13th August 2012 12:45 AM

There are lots of coupling capacitors in this amplifier - there is one between each stage. A coupling cap couples audio from one stage to the next. Examples C7, C13, and C18..

Power supply filter and decoupling caps provide clean dc to the various amplifier stages. C1 is the supply filter cap and MUST be replaced.

C3 is the cathode bypass on the output stage and also needs to be replaced. Any other electrolytic capacitors in the amplifier should be replaced.

Replace both 6V6 preferably as a matched pair.

I suspect you need to do a little basic reading to understand tube audio electronics a bit better before you proceed further..

I hope you understand how to work safely on vacuum tube gear - if not please read the newbie and safety threads in the Tubes / Valves forum before going further.

kevinkr 13th August 2012 12:50 AM


Originally Posted by JoelKnauber (
to be honest i'm not entirely sure what the coupling cap is.

<picture snip>

is that the large 400ohm 7w one after the rectifier?

I haven't turned it up without the cab plugged in.

The 400 ohm 7W device is a power resistor. C1A-D is the filter capacitor.

I'd recommend you find someone local to mentor you regarding tube guitar amps and the conversion process - possibly a member local to you.

JoelKnauber 13th August 2012 12:54 AM

Okay okay, yeah I know which ones you're talking about. I've read a lot about preamp stages, and I am a mechanical engineering major in school and i've had a couple basic electronics courses.

I DO know about discharging caps. and I'm always up for learning more. but I'm about to order a new set of tubes for it, based on how it didn't kill the tube which appears to be good, do you think i would be safe with putting new tubes in it?
It fires up and idles just fine, nothing out of the ordinary..

JoelKnauber 13th August 2012 01:09 AM

also, could you explain why certain caps MUST be replaced?

kevinkr 13th August 2012 01:15 AM


Originally Posted by JoelKnauber (
also, could you explain why certain caps MUST be replaced?

All capacitors have a finite life span, and electrolytics in particular deteriorate over time whether in use or not, and they fail catastrophically - usually by shorting (or partially shorting) which sometimes (frequently) results in a big bang and when it doesn't the short usually takes out the power transformer with it.

Coupling capacitors in this time frame were usually made with paper dielectric, and since they are not generally very hermetic the dielectric absorbs water over time and becomes quite leaky - this results in shifts in operating points that prevent a stage from performing its intended function, and in the case of an output stage usually results in the destruction of the output tube connected to it.

Would you drive an antique car with 50yr old tires or replace them with new ones?

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