High Power Twin from Peavey Vintage Iron

[lord preset]

I haven't done a build in a while but a friend passed on a 1970s Peavey Vintage chassis to me that wants a new start in life. This was one of the 1st tweed clones, cosmetically at least - internally it was a hybrid SS front end with a 100 watt 4-6l6 power section. After gutting it I am left with a nice steel chassis and some stupid heavy transformers. Chassis space is relatively limited so I want to keep it simple. I thought about a Dumble Small Special but I've done two versions already, so the next most obvious thing I could think of was some version of a 5f8a circuit with some necessary changes. No center tap so a SS bridge rectifier is called for and the original design lacks a bias pot. The Peavey Vintage PS circuit put out around 490v B+, quite a bit higher than the 58fa calls for. At least initially I don't want to mess with zener/Mosfet schemes for dropping B+, so I expect that the amp will be brutally loud and clean. I know it won't sound like a tweed Twin, but I have a friend who likes Loud and Clean and doesn't mind Heavy. We will see.

[lord preset]

Btw, anyone have any experiences good or bad with adding a Lar Mar PPIMV to a Tweed Twin or Bassman? The preamp stays pretty clean so I assume it would be only moderately useful but it might help the amp open up a bit at low volumes.

[xtian]

I'm distracted by your cardboard transformers. Or are those made from manilla envelope material?

[vibratoking]

They are the inverse of a black body radiator. Keep the heat inside as much as possible.

[xtian]

Sorry, don't mean to tease. Much. It's going to be a great build!

LARMAR is cool on JTM45, which is close to Bassman. Go for it.

[lord preset]

I have an ISO9001 documented process for selecting the color of paint to go on transformers.

1. Open garage cabinet where spray paint is stored.
2. Select the can closest to the front of the cabinet.
3. Shake the can.
4. If there is enough paint in the can to paint the transformer(s) go to step 6
5. If there is not enough paint in the can to paint the transformer(s) select the next closest can and go to step 3.
6. Use the paint.

Thanks for the LarMar info.

[lord preset]

Progress. 1st time I have worked in a tweed style chassis and I can't say I'm a fan. I normally do overhead heater wires but it seemed like it would be a less than great idea here as flying them over the tubes would bring them closer to the board. Parts mostly culled from the bin. Unfortunately I can't use the 6L6s that came with the donor amp. They were Russian 6L6GBs that tested bad, presumably because they didn't like 500V on the plates. I will be trying some Shuguang 6L6GC cheapees instead.

[lord preset]

Back from vacation and the two conductor shielded cable needed for the master volume had arrived so I was able to finish up the build. Here are some pre-startup pics to document what it looks like before the smoke stains and charring.

[lord preset]

So I went through start-up. No bright bulbs without tubes. Check. No bright bulbs with tubes. Check. I had it connected to a dummy load with the LarMar master down. As I brought the master up the OT started making all manner of unnatural sounds from growls to whines depending on the position of the master volume. I was concerned since the OT from a donor amp of unknown provenance. Feedback from switched OT primaries was my 1st thought but I've never actually encountered this before and was doubtful that the it would make the OT mechanically vibrate. Well it did and it does. Flipping the primaries did the trick. Other than that it fired up just fine. Reasonable voltages, not stiff sounding which was a concern with SS rectification and 495v plates. The master volume has a very narrow
useful range on the dial but otherwise seems pretty usable. Only a few questions:

1. The Chinese 6l6GCs I bought on eBay were sold as being carefully current matched, but they were most certainly not. What is the world coming to? If we can't trust eBay sellers who can we trust?

2. Why did no one tell me that Tweed Twin volume controls are interactive? I thought something was horribly wrong. No one tells me anything.

[rooster]

I built one a few months ago using a tapped HPT PT from Heyboer and a large 4-8-16 tapped vintage Triad OT. After trying to do a gig or two running a quad of JJ 6L6s at 435VDC on the plates and a GZ34 Mullard recto, I am now using just a pair with the lower voltage taps (407 VDC) and using a diode plug.

I did use separate cathode caps and coupling cap values on V1 (12AX7), somewhat following a Plexi path on the front end. I also purchased a foldable Lucite shield for the front of the cab(s) (1/12 or 2/12) and gig with it often. I really loved how it sounded running a quad and the recto (or without) straight into a 4/12 without pedals but it was deafening. Pulling a pair of tubes and running with the Lucite panel and smaller speaker cab has made all the difference. It's a great sounding amp, no doubt.

OK, back to you, haha, with 500VDC on the plates I am thinking your amp is louder than hell! Man, please tell me how that amp gigs.

[lord preset]

Well it IS loud, but a couple of things make it more manageable. 1)I have a usable master volume which helps a lot. and 2) I am running the 4 ohm Peavey OT into an 8 ohm load which knocks the effective wattage down a bit. I have found that fiddling with the interactive volume controls and and the master I can get some really full sounding crunch and grit at reasonable volumes. We'll see how it gigs, but I won't be doing it. This will be one heavy mofo of a combo. The cabinet is light pine but the chassis weighs a ton as do the 2 90 watt celestions I am using ( because I had them ). Hopefully this is going to a friend who's back is stronger than their common sense.

[lord preset]

Finally got around to finishing this up. Rather than building a cabinet I found a deeply discounted Mojotone cabinet that a builder custom ordered but apparently could not use. It's HP Tweed Twin cab with a chassis cutout sized for a LP Twin. It turned out to be perfect for the Peavey chassis as it has no tube rectifier and was thus shorter by just the right amount. Speakers are Egnater-specced Celestion 80s.

[xtian]

Siiiiiickk!!!

It's so fun when you get them zipped into their fancy suits, isn't it? Looking sharp!

[rooster]

Yeah man, that looks really nice! You got lucky on that cab, too, it's a perfect fit!

Here's two pics of min, BTW. I built and covered it. Note the hole in the bottom of the cab under one of the power tubes - this gives me room to use a Eurotubes bias probe. Also note the four (you can only see three) holes behind the grill cloth, a cooling idea.