Peavey Windsor Studio Mods and Problems

[ttenroc]

I just got a Windsor Studio in a trade on Craigslist for and old marshall solidstate + a little $. I brought it home, plugged it in, and immediately decided to mod it for better tone. I found this thread after digging for a while.

First I removed the power sponge, which opened it up a lot.

Next I did a tube swap. I happened to have a brand new 6550 laying around and they yield a little more of a bell tone and just a bit more bass than 6L6s and EL34s imho.

I still didnt get the tone or bass response I was looking for so I swapped the Blue Marvel with a vintage 60watt 8ohm Yamaha made speaker out of my 1972 Yamaha Electone organ that went out of commission recently.

Finally I got the tone I'm looking for out of my Windsor Studio. Warm and creamy wide open tone with that early 70's growl.

I played on it about 20 minutes... I was going though an EQ and preamp/ power amp settings test to see what sounds I could achieve out of my new favorite sounding amp. I turned the preamp down to 3 and went to set the master to 7 and the volume simply cut out.

I turned the amp off immediate. Checked the power tube to make sure it was seated properly and a burned electronics kinda smell. I assumed that I maybe had a bad tube. and swapped it with the 6L6 that was in the amp when I got it, and the sound returned.

I played through the 6L6 for 5 or so minutes same thing happened... I have noticed that If I turn the preamp and the Master vol all the way up I still get the loud hiss as though the amp doesn't have anything plugged into the input when Im not playing, and very faint sound from my Les Paul when I strum if I am plugged into the high gain input.

I also tried to use the effect loop's send into another amp to determine if the the problem is in the output section. No signal... so I tried taking the signal from the effects loop send of the other amp into the windsor's return...Nothing still...

I understand that I may have gotten a raw deal on CG, but I'm pretty handy with a soldering iron so If its a bad cap or resistor I can change it.

Any Ideas?

Here is a schematic http://www.mediafire.com/?jqlongrzfy1 .
__________________

Jim P or any other gurus ???

[jim p]

I have some ideas on maybe getting closer to what the cause is. One thing you can try is to play through the effect loop return that will bypass the first three tube stages the volume pot ect. You will be using the volume pot on the guitar as a volume control when you do this. Another thing would be to turn up the reverb and bang on the tank lightly to see if you get any sound from the reverb tank when the amp cuts out the reverb return is after the volume pot.
Because it is temperature dependent with the amp open when it fails you could try blowing cool air at different parts of the amp to see if the sound comes back on. A hair dryer on cool setting would work well for this. With the hair dryer you can do the opposite and use it to heat parts of the amp to cause a failure also just don’t overheat anything.
Look to see all the heater elements are glowing in all the tubes the preamp tubes are in series across 24 volts while the power tube has its own 6.3 volt supply.
Failing with heat would tend to be an electrolytic cap or solid state component such as the op amp driving the reverb tank. If the op amp fails it might be able to swamp the signal right at the volume pot wiper as a guess.
You can also connect the send out to another amplifier to see if you always have signal there if it cuts out here the problem is in the preamp section. If it is always good then it would be the power stage V2, V3, output transformer, impedance selector switch, ect.
Just in the off chance it is a bad impedance selector switch move it to a different setting it is ok to have an impedance mismatch of a factor of two so 8 ohms at 4 ohms is ok ect.

PS I was just rereading your post and missed the part about trying the effects loop in and out. I gota have some coffee before I look at this stuff. Anyway using the loop in try swapping the two 12AX7 tubes if V2 is bad you will get no sound out of the amp no mater where the input signal is applied. If V1 was bad the loop would work but the front end input would not. Also keep in mind the 12AX7 heaters are in series so if a heater fails in V2 or V1 neither will work so look for the heater element glow on the tubes.

[ttenroc]

Thanks for your help Jim.
:thwap I'm a little bit embarrassed to say that a bad guitar cable ended up being the problem. I never even thought about switching chords until dong the effects loop tests you mentioned and I happened to change chords when switching between send and returns between amps.

I always kept the reverb way down or off before my power chord shorted out. You drawing my attention to the reverb unit has uncovered a different problem though... when I turn the reverb knob up past 6 i get a terrible high pitched ringing that sounds like a microphone feeding back. I have also noticed that there is no increase of reverb saturation as I turn the knob up. Does this means I need a new tank? I can just take it out/bypass it if I need to right?

After playing on the amp a while this morning i have noticed that as I lower the gain on my preamp the bass is also lowered. If I set my peramp volume to around '8' I get almost perfect EQ control. ie... increasing and decreasing bass, mid, and treble yields a very pronounced effect on on tone. Is there a mod that will give me that kind of tone control when my preamp is set to '3'?

Is it as simple as lowering the resistance value of R3 to around 33k?

[jim p]

It has been awhile since I modified the Windsor head but this amp also has a lot of brightness and gain in the preamp section. You could get more bass by just removing some of the brightness and this would improve the clean headroom. With all the high frequency gain they added C14 across R34 to cut it before the power tube. You could lift C27, C28 and also C5 if you want, this will lower the overall gain of the preamp section but you still have C37 and C36 for brightness (at approx 800Hz). On the reverb check the RCA cables that go to the tank, the return signal cable is also the ground for the tank so check for corrosion. If you have another RCA cable hooked to the TV or something borough it to try a different cable. If you can hook to an external speaker instead of the one in the combo and the reverb feedback goes away then it is acoustic pickup and you will need to try to isolate the tank better. You can also take the tank out of the cabinet and put it on the floor or a stack of books or whatever to isolate it from the speaker as a test. You could increase the value of C35 to roll off the highs into the tank try a factor of two to 33nf if it is still a problem go to 42nf. For more reverb you could change R50 to 100k you will need to change C44 to 1nf to 500pf also.

[ttenroc]

Removal of C27, C28 and C5 were Just What the Doctor ordered! :thumbsup
And You were right about the rca cable connections being bad. A Quick unplug and replug fixed it.
Thanks again Jim. I am really glad there are people, like you, with a vast wealth of knowledge who are willing to help another person with projects like this one.

[jim p]

I would guess you have the schematics for the Peavey Windsor Studio already? If not they are at a link in this thread above. Looking at the schematic having the volume control all the way down should eliminate the preamp as the source of the noise but you could remove all the 12AX7 preamp tubes and run the amp to see what the noise is as a test. This head being a class A like the Valve Junior has the same problem with plate supply noise so what lowers the noise on that would also work here. On the schematic they have three 100uf caps after the bridge which is plenty of capacitance. But part of your problem maybe the line voltage frequency if you are 50Hz. One thing you could try would be to add a resistor for a Pi filter after the bridge rectifier. If you break the connection between C10 and C43, C47 then install a 5 Watt 200 or 400 ohm resistor that will lower your plate supply by approx 16 to 32 volts but should help filter the plate supply. A BIG CAUTION with the standby switch open their is no bleeder resistor on C10, C43 and C47 so be CAREFULL you should put the standby switch in the on position with the power off to discharge those caps and test with a meter. My voltage drop on the added resistor is using an 80mA supply load as an estimate. If you want to not loose as much voltage as you do with a resistor a 1 Hennery or greater choke capable of 150mA or more instead of the resistor would do that. Another way to go would be to install a variable voltage regulator for the plate supply of the output tube this will drop 10 volts or so also but with good filtering on the gate of the mosfet there will be almost no ripple on the plate supply.

Here is a schematic mark up of the mod http://i1020.photobucket.com/albums/af330/vactube/WindsorStuidoPSmod.jpg

[bluesXL]

Many thanks Jim! I will let you know if the mod works, however first I will study the powersection of the amp for safety reasons...

After looking at the schematics once more:

When I removed the power sponge of this amp (attenuator), I soldered the black wire to a chassis ground near the power supply (other side of the chassis). I'm wondering if this could cause a hum. As I can see it, the ground-wire to the attenuator could also be removed entirely, as the speaker is already grounded to AGND2B, which apparently is a different ground than e.g. AGND1 (???):confused:

Who has thoughts on this?

By the way: one strange feature of this amp is that bass is kicking in when you turn up the pre-amp volume... Is that normal / logical?? I get a nice 'clean' and more or less bright tone until 5 o' clock (thanks to removal of c27,c28, c5), after that overdrive kicks in, but also bass. After that, to my ear, the sound is then getting a bit muddy.

[jim p]

Just occurred to me this morning but another source for the noise could be the reverb so check the RCA jacks for corrosion. Also the coaxial RCA cables should not pass near the power transformer.

Another improvement would be to add a 20 ohm 1 watt resistor between the bridge rectifier and the first filter cap C10. This will change the ripple waveform on the filter cap removing the fast rise time due to the low impedance of the forward biased diodes, also it will reduce the RMS current in the transformer secondary.

[bluesXL]

Ok, thanks but this is not the problem I think; I have disconnected the reverb unit entirely because the sound is not pleasant; maybe I'll try to dampen the springs somewhat with cloth or parafin (just a thought). Most probaly I will use an external device e.g. t-rex reverb using send-return.

[ttenroc]

I modded the tone stack on mine to resemble marshall's design. You still get more out of your EQ as you progressively increase gain, but it gives a bit more to you in the bass and mids than stock. I used this tone stack calculator to help me decide on the final values.

http://duncanamps.com/tsc/index.html

Here is a pretty informative article on tone stacks if you are interested...


http://amps.zugster.net/articles/tone-stacks

Also... Have you done some tube swapping recently? could be a dirty tube socket... I don't know about the noise problem that YOU are having, but I began to have issues with a dirty power tube socket shortly after my initial mod conversations with Bill. It sounded like static in the back ground. I took a big rubber eraser and tapped on the the power tube while it was happening. The sound should dissipate or even quit immediately when you jar the tube. I put some wd40 on a q-tip and saturated the pins of an old power tube then inserted it into the socket repeatedly while cleaning off the pins between each insertion. When
finished I washed my hands thoroughly and put the good tube back in. This may not be the preferred method of cleaning but it worked for me.

Just a thought.

[bluesXL]

Just to let others know: I found out that the adapter of my line6 pod was causing most of the hum in a very curious way (at least to me!):

situation:
I have 2 power points / contact boxes in my room that are both independent (seperate fuses, don't know how to explain this differently)

the experiment:
1. peavey and adaptor podxt in same power point = a lot of hum
2. peavey and adapter podxt in different power points = large reduction of hum

does anybody understand how this works??

[Ch0jin]

Just out of curiosity, is the Peavey power cord a 3 prong and the POD adapter a 2 Prong?

I'm just having a guess here but it sounds a LOT like an earth loop issue. A lot depends on your power wiring scheme. Here in Oz all power points are 3 prong jobs (we have whats known as a MEN or multi earth neutral system) so every power point has a ground and if your device has a three prong cord, it's grounded too. You can run into problems using a 2 prong device like a power brick and a 3 prong amp because the amp is grounded, but ground on the DC jack is actually floating about true ground meaning there's a chance hum can be introduced.

Anyway, I'm just thinking out loud. I'm an electronics guy, not an electrician :)


OH WAIT

When you say it hums both plugged into the same outlet, it's not as simple as a bad power board or double adapter is it? I know a few guys here have had big trouble with cheap power boards and hum.

[bluesXL]

Thanks for your reply! Indeed the podxt is a 2-prong (without earth). The adapter of the podxt is many times interfering with other equipment, e.g. my guitar.

The hum I get without adapters in the same outlet as the amp is very low; not really audible when playing at living room volume, yet audible when micing the amp at low volume.

I guess that a pro recording egineer would record an amp at loud volume and use a gate (and use a better amp!). I can't record loud, and I like the tweakability of this amp, so basically I need to live with a little hum.