Windsor Studio bass/tone mods

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65 SG
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Windsor Studio bass/tone mods

Post by 65 SG » Thu Sep 21, 2017 1:06 pm

Been away from the forum for awhile but recently picked up a used Windsor Studio. It has features I like very much (Class A1, real reverb, loop bypass, power soak, isolated / transformer-coupled XLR out) and I figured if it was a Peavey, I'd just love it...but my initial impression was underwhelming...to say the least. Did some research and found that many users complain about a lack of bass and/or a large variation in bass with gain/volume change, and an overall brittle or thin sound. And nearly impossible to get a usable clean tone. And how hard it is to change the power tube without removing the power soak and/or speaker. Andf that the Boost switch is one-dimensional and not very useful.

I saw some other mods (elsewhere) that involved removing the sub-microfarad cathode bypass caps, which reduces stage gain, and removing high pass treble boost caps and treble bleed caps here and there. These changes would help reduce treble somewhat but IMO are not nearly enough to get the bass response up to where the Bass control doesn't have to be dimed (dozened? the faceplate goes to 12). There are other changes floating around out there that are more perception than science, like swapping out the "puny" ceramic tonestack caps with "better" film capacitors. I'll go to my grave agreeing with Peavey on this one and see no need to alter any tonestack components.

The main "fix" as I applied it was to add larger electrolytic cathode bypass caps in some of the stages and to strip out much of the mystifying tone shaping components (or, design band aids, as I see them). For example added HF bypass caps after a high pass filter. Why not just tailor the high pass? Or remove it completely?

Anyway, after a couple hours modding the amp the bass content is up considerably and I think I have rescued the tone of this amp - which puts it on par with my VK (which it shares many design similarities with) and my C20. V1 tube changes help a bunch and allow the amp to be set up for high gain (as designed) or lowered for more clean headroom. If anyone's interested in the specific circuit/tube mods, chime in and I'll flesh out this thread into a nice mod guide. If not, I won't bother :)

The other thing I did was remove the power soak, so I can more easily play with different tube selections. I may relocate it amidships later. Maybe. And I pulled the heavy and over-motored Blue Marvel driver (no need for this much magnet / weight and voice coil area in a 15 watt amp) and dropped in a 1976 (1966?) CTS alnico 16 ohm stamped frame driver I had in my junk pile. That was like fitting the shoe to Cinderella's foot, a perfect match. This speaker is clearly more efficient than the BM and added a bit more low end punch. Plus, it was definitely broken in! Finally, I trimmed off the codpiece. Never cared for that Edsellian aesthetic.

Next step is to repurpose the "Boost" switch in a JCM type mod, as I did for the VK. Goal is to approach the versatility of a 2-channel amp
'03 SG Special, '08 LP Special DC, '60s Dano/Silvertone, Epi Moderne, Epi EB-0, '60s Telestar semi-hollow,
Classic 20 w/6V6 mod, much modded VK112, Windsor Studio combo, Transtube Envoy II, '61 Reverberocket

KTB
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Re: Windsor Studio bass/tone mods

Post by KTB » Sat Sep 23, 2017 7:17 am

I've been away for awhile myself so quit stalking me man lol !

There is a guy on this forum that I sware did almost every mod you just talked about but can't remember who it is. I know exactly what you mean about all the frequency shaping steps and how they effect something else down stream if you eliminate them or mod them. Imo it's very difficult to mod because of the layout of the boards and trace cutting you have to do because the components share the same pads and routing. It's almost more economic to install your own PCB and route it like you want and know that thought crossed your mind. Anyway glad you got it sounding better and will pass it on if anyone needs a Windsor mod tone mod.,

KB

lionheart
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Re: Windsor Studio bass/tone mods

Post by lionheart » Mon Sep 25, 2017 3:06 pm

I put a Celestion G12H Anniversary in mine. Smoother sounding than the Blue Marvel and way more efficient with noticeably more bass and depth to it.
I too removed the Power Sponge and felt that it's removal improved the overall tone somewhat. How exactly I don't know - possibly due to certain frequencies still seeping to earth even when the Power Sponge is set full on.
Tubes I settled on a Svetlana EL34 and a pair of EH 12AX7s - a big fat Marshall type tone but I really would like to get more bass out of it as mine is maxed all the time and it's still not giving that elusive bottom end thump I yearn.
I tried it into a 2x12 closed back Marshall cab (2 x G12T75s) and the bass was still lacking compared to other amps so it is obviously an inherent circuit issue with the Windsor Studio.
A likeable little amp for classic rock (particularly good at Cream impersonations with an SG) but needs way more bass !!!!!
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65 SG
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Re: Windsor Studio bass/tone mods

Post by 65 SG » Fri Sep 29, 2017 12:55 pm

Probably the one single mod you can do to noticeably increase bass on this amp is change C52 to a 10uF or larger, 50v electrolytic cap. The problem is that the V2B driver stage is after the tone stack but just can't reproduce lows well enough. Upstream, C5 and C28 can be increased also, but you can take this good thing too far and wind up with a muddy tone...

One thing you might also consider especially if using an EL34 in this amp is changing the cathode resistor. This mod extends tube life and increases clean headroom somewhat (at the expense of power stage harmonic content).

With the plate voltage used in this amp, an EL34 wants to idle at about -34V grid bias. The 400 ohm 10 watt in the original design results in 85mA cathode current - extremely hot biasing. There's 475V on the plate of this tube and 34V at the cathode which works out to (475-34)* 0.085 = 38 watts. The EL34 is rated for 25 watts plate dissipation and running it at 38W contributes to short output tube life - it's on the verge of redplating.

I changed R38 to a 600 ohm 10 watt which lowers the "no signal" idle current to ~55mA, or ~24 watts. This also reduces the load (thus ripple) on the power supply and lowers the DC current in the output transformer which helps bump low end slightly as well - minimum core size is heavily influenced by the plate current in a Class A amp and reducing the DC bias moves the core further from saturation at lower frequencies which in turn allows higher efficiency down there. Anyone who's considering an expensive transformer transplant should try a bias mod first.
'03 SG Special, '08 LP Special DC, '60s Dano/Silvertone, Epi Moderne, Epi EB-0, '60s Telestar semi-hollow,
Classic 20 w/6V6 mod, much modded VK112, Windsor Studio combo, Transtube Envoy II, '61 Reverberocket

lionheart
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Re: Windsor Studio bass/tone mods

Post by lionheart » Fri Sep 29, 2017 1:27 pm

@ 65SG
Thanks for the reply and your valued suggestions.
I'll certainly give the cap you indicated firstly a tweak in the right direction.
As for EL34s being on the verge of redplating with this design of power amp - that well and truly confirms my suspicions that it was running a little on the hot side as the EL34 in this amp is physically way way hotter to the touch than the two I have in my Marshall DSL.
Never mind naming it the Windsor, they should have called it the Peavey Chernobyl it's that close to melt down !
The Windsor Studio needs the addition of a real big kick in the butt so to speak as it's only real down side is it's almost complete lack of bottom end.
My little Blackstar HT-1RH Head produces way more bottom end thump through a closed back 1x10 cab than the Windsor.
The Windsor however has a lot lower noise floor and an FX loop plus full tone stack to it's advantage.
If your expert guidance gives me the thump I need, I feel the Blackstar might just be shown the door - no point in keeping an amp with no FX loop that hisses like a punctured tyre on higher gain settings !

forgivenman
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Re: Windsor Studio bass/tone mods

Post by forgivenman » Sat Feb 10, 2018 8:46 am

I’ve changed the speaker and the power the’s on the amp and it really made a big difference. The speaker I put in was a WGS cream Back. The difference in tone was incredible. I also run a 6L6 in the power tubes lot. These were non-invasive changes they made a huge difference in the time. Unlike a lot of people I didn’t buy this to use for a clean platform. I wanted it to roar Or do mid gain crunch at the least. The new speaker increased the bass response significantly.

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