Peavey Windsor100 head power section 6550 mod...

[ttenroc]

Does anyone know what modifications I need to perform on my Peavey Windsor 100 watt amplifier to make it operate on 6550/kt88 power tubes. I am using the amp as a bass amp and need more headroom to power my Ampeg 2X15 cabinet .

I understand that the Windsor was designed to be a high gain amp, but I have already performed a clean mod and changed the tone stack to get the amp voiced the way I like it as per these threads(http://www.thefret.net/showthread.ph...Peavey-Windsor , http://www.talkbass.com/forum/f15/pe...s-bass-522387/ ). Please note that I did not do the power section mod that Bill did on his amp.

After the preamp mods I can get some great clean tones up to around 2 o’clock the pre amp. Now I need more from the power section to combat the power tube distortion that I get when I turn my master volume up. It gets dirty around 1 o’clock… Is the 6550 mod as easy as changing the tubes and adjusting the bias pot or do I need to change some capacitor/resistor values to make it work? If so, which caps/resistors and what will values be? Will I need to change my power transformer?
Here is a link to the schematic:
https://rs312l3.rapidshare.com/#!dow...r.pdf|3257|R~0

[tunghaichuan***]

A couple of things come to mind:

If you have to ask which resistors to change, I'm guessing that you haven't worked on a lot of amps. The Windsor is a 100 watt amp with high voltages present inside. That amp can kill you dead in a hurry if you don't know what you're doing.

The other thing is that being a guitar amp, the OT is probably not suitable for bass. It probably is only spec'd down to 80 Hz or so. A four string bass goes down to what, 40 Hz? So it may overdrive even if you do put in different tubes and rebias.

I would take the amp to a tech and discuss it with him/her. If all that needs to be done is to change out a few resistors, it shouldn't be too expensive, if you already have the power tubes.

From the schematic it looks like you'd need to change out one or two resistors in the bias supply and the grid leak resistors on the power tubes. Not complicated, but dangerous.

I don't mean this to insult you as I don't know your background.

Does anyone know what modifications I need to perform on my Peavey Windsor 100 watt amplifier to make it operate on 6550/kt88 power tubes. I am using the amp as a bass amp and need more headroom to power my Ampeg 2X15 cabinet .

I understand that the Windsor was designed to be a high gain amp, but I have already performed a clean mod and changed the tone stack to get the amp voiced the way I like it as per these threads(http://www.thefret.net/showthread.ph...Peavey-Windsor , http://www.talkbass.com/forum/f15/pe...s-bass-522387/ ). Please note that I did not do the power section mod that Bill did on his amp.

After the preamp mods I can get some great clean tones up to around 2 o’clock the pre amp. Now I need more from the power section to combat the power tube distortion that I get when I turn my master volume up. It gets dirty around 1 o’clock… Is the 6550 mod as easy as changing the tubes and adjusting the bias pot or do I need to change some capacitor/resistor values to make it work? If so, which caps/resistors and what will values be? Will I need to change my power transformer?
Here is a link to the schematic:
https://rs312l3.rapidshare.com/#!dow...r.pdf|3257|R~0

[ttenroc]

Tunghaichuan- I appreciate your concern with safety. You are right to assume that I haven't worked on a lot of amps. Guys like you keep folks from shocking themselves to death due to ignorance or carelessness.

However I do understand the lethality of the voltages stored within the various caps throughout this amp, and have been shown by one of my EE student friends how to test and safely discharge caps with a MM. He is also a member of my band and is present for most of the work I perform.

I have been talking to him about this project and also has reservations about the OT, but he is so busy with everything else that he hasn't had time to devote to sitting down and figuring out answer or consult his professors.

I am beginning my freshman semester as an EE Tech in a few days because I love doing this kinda stuff... I have built, modded and repaired a few effects pedals and performed the mods/repairs to a Windsor Studio(with the help of Bill P and an email reply from peavey's tech support team) and to my Windsor 100 amp as mentioned in my OP. I have found forum to be vast wealth of folks, like you and Bill P, who know the answer to any question about electronics that can be asked.

It is because of the things stated above and that the last time I brought my local amp tech my amp it cost me a couple of hundred $$ to have him "repair" my previous amplifier which still has the same issue it had when I took it to him sans the new tubes that he swore fixed the issue. Turns out all he did was put my preamp and power tubes in a tubes tester and changed the bad ones he didn't even plug it in and turn it on to see if the tube swap fixed the issue. He didn't even look to see what the problem was that caused all my tubes go bad. I got home excited to have my Traynor amp back and fried my circuit board as soon as I powered it up. When I confronted him about it he said that he didn't have a guitar cab with a high enough wattage rating to plug my amp into. I would have gladly dropped my bass cab off for him to test his work.
It is now sitting in the corner of the room waiting for me to learn all I need to learn so I can fix it my self. I'm sure he has all sorts of knowledge, but he was lazy and careless with my amp, and therefore with my money, so I will never "donate" to his business again. And not only will I never do business there again, but after I get graduate I plan on getting authorized by every major amp manufacturer and opening a shop right across the street from him with a sign that says I repair what he "fixed" for free.


The long story paraphrased is I am determined to do this project at my house with my soldering iron.

Sorry for the amp tech rant...

[Bookkeeper's Son]

The few things I've read about residual tube amp voltages state that injuries and fatalities directly due to the shock are extremely rare, but that secondary results from reaction to the shock (dropping, bumping, etc.) are more likely. Where does the line into urban legend get crossed?

[jim p]

What are you trying to do by changing over to 6550 over EL34/6L6 as the output tubes? You cannot make a 200 watt head out of a 100 watt head if that is what you are thinking. The output transformer and power supply could support a two tube 6550 set up instead of the four tube EL34 configuration, but not four 6550 tubes. The power supply would have to be capable of supplying twice the current, thus twice the power to support four 6550 tubes. Also the output transformer would need to be able to handle the higher current without going into saturation.

[ttenroc]

Bill you're right about my desire to make this into a 200 watt amp and I'm willing to change the transformers from deceptecons to autobots to make it happen if it will indeed work. I have alread test fitted the 6550s on the chassis and its a tight squeeze but they fit.

I have a larger PT now 600watts 60hz 120vac that I harvested from an old Peavey Stereo Chorus 400. Would that be large enough to power four 6550s?

Also it seems like this am may already be designed to saturate the OT early since I'm getting power tube distortion with the master set to around 2 o'clock and the pre at noon. The amp cleans up tremendously as I turn the master down, but then I cant keep up with the rest of the band. I don't mind some growl but I wanna control that with my preamp setting...
Yes. The Band is way LOUD

Any ideas on where I can find an OT better suited for my needs?

Would there be a benifit in running 2 6550s over 4 el34's?

[ttenroc]

Would this one work?
http://www.parts-express.com/pe/show...number=122-155

Hammond
1650T 120 Audio Watts, 1,900 ct, Push-Pull Par., (4 or 6 Tubes) 6L6GC, 5881, EL34, 6550B, KT88

[jim p]

In looking I have found these two output transformers http://www.triodeelectronics.com/a451.html this one for a Dynaco audio amp so the low frequency response should be good. Here is the manual for the Dynaco amp http://www.dvq.com/hifi/images/dyna_mk6.pdf it has a schematic in it and the data sheet for the tubes http://www.mif.pg.gda.pl/homepages/f...168/8/8417.pdf Also this transformer http://angela.com/hammondoutputtransformer1650t.aspx a Hammond 1650T. For a bass amp you could consider wiring up the output as Ultra Linear http://www.ampbooks.com/home/amp-tec...y/ultralinear/

And this posting http://oestex.com/tubes/ulo.html

[ttenroc]

Thanks for the suggestions. In your opinion which is better.

I have a question about the ultra linear style of output wiring... When these articles talk about 43% of primary winding, which is the point minimum distortion, does it mean that there will be next to zero distortion in my power section? How will this translate to volume? Would the amp be as loud as it would be if it were wired the way it is now?

[jim p]

Relative to which transformer would be better I can’t say. I haven’t used either one and the specifications don’t give enough detail to say one way or the other. If you go by the heaver the better for bass you would need to find out the weight of the one they have at Triode electronics. The one they sell at Triode is probably made by Magnetic components and as far as I know they make good transformers that people are happy with. As far as the ultra linear connections if you look at the schematic for the transformer at Triode http://triodeel.com/dynacoa451.jpg there are five taps. The plate supply goes to the red wire, Blue/white to one set of plates with green/white to their screen grids. Then blue goes to the other set of plates with green to their screen grids. The screen grid taps on the transformer are at 40% of the windings for each set of tubes. If you look at the data sheet for the 6550 http://www.mif.pg.gda.pl/homepages/f...35/6/6550A.pdf they have tables of the specifications for both pentode and ultra linear along with static curves. The plate impedance of an ultra linear amp is lower than a pentode due to the feedback created by connection of the screen grids to the output transformer. With lower primary impedance (Plate) the amplifier will not be affected by the changing impedance of the speaker as much so less distortion. There is a very large change in the impedance of a speaker in the range of 80-120 Hz due to the resonance of the speaker. The resonance is relative to the damping due to its surround and the added damping relative to the speaker enclosure. Also the impedance of a speaker is very low outside of resonance at the low frequencies. An 8 ohm speaker is 8 ohms at 1 kHz due to inductance it is less than 8 ohms at say 200Hz and more than 8 ohms at 2 kHz. By the data sheet the power output with ultra linear and pentode connections are almost the same.

[ttenroc]

So with that info I can hypothetically just replace the OT in my current set up, without changing the tube arrangement, and have clean power with my amp dimed?
I was intrigued by the output power of the 8417 tubes on the link you posted( http://www.mif.pg.gda.pl/homepages/f...168/8/8417.pdf )so I did some looking into those as an option. turns out they are very rare and expensive, but folks are suggesting tungsol KT120's as a replacement. http://www.tubedepot.com/ts-kt120.html
Looks like a pair of KT120's can put out 150 watts under the right circumstances. Do you think I safely can use a pair of them in my Windsor?

[jim p]

I was just posting the specifications of the tubes in the Dynaco amp so you could see the similarity and differences between them and a 6550. I don’t see any reason you could not go with using the 6550 as the power tubes for the amp. Also the more common and available the tube the better as far as in the future to maintain the amp. As far as ultra linear versus pentode the link to the web page on ultra linear should give you an idea of the differences. It is not that hard to rewire the amp one way or the other , just need to have a transformer with the screen taps for ultra linear. As before the stock supply may have a problem with the greater current requirement so you may need to replace the power transformer and add more filter capacitance.

[ttenroc]

Sorry for not posting a reply for so long. Here is how I ended up solving my problem.
First I changed R-5 in my head from a 1 to a 2.2k resistor(Bill did this in a windsor mod thread, but changed R5 back to stock resistance in his mod to retain gain levels). It gave me cleans for days in the preamp.
After looking for a while at OT's I decided that a whole nother Peavey windsor can be bought for the same price as a new OT, so I found another one on craigslist and traded my old TS-9 pedal even for it so no cash lost there. I then stacked the two heads together and ran the effects loop line out of the modded windsor into the return new one. This allows me to use the new amps power section as a "slave". When I change any setting on the top amp it comes through the slave amp sounding exactly the same. I also rewired my bass cab so that each 15" speaker has its own input jack. I can now power each speaker independantly and give it the the full power of each amp which equals tons of head room. I can still get distortion, but ONLY when I dial it in.
Fixing this problem gave me enough cinfidence to tackle the broken Traynor amp that I ranted about in the earlier post. Turns out that all I needed to do was open it up and cut the burned traces and hard wire the 3 broken conections that were arcing out. Now the Traynor works too. Thus reuniting me with my beloved fire breathing Traynor and leaving me with 2 great backup amps.
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Bill, thanks again for your advice. You are a very knowladgeable and helpful individual.