UL Bassman 70w-style design, input appreciated!

[DenaliJoe]

Greetings all!

I'm hoping I can solicit a few thoughts regarding my attempt to build a single channel version of a Bassman Ten 70w amp. Yes, one of the dreaded ultralinear designs.

Why? I have a gigging musician friend who needs a lot of (ideally warm) clean volume in a small package. "Small" eliminating a Twin Reverb, obviously. I'm making a head, not a combo.

On hand, I already had one of the Triode Electronics A-431-S OT's (for a long deferred Dynaco restoration)- so initially considered one of the 2xKT88 designs that have been discussed here on the forum (Marshall Minor, Interstate 88, etc).

Once I looked at my parts on hand (especially the Triode OT) and the available budget, I realized it's more realistic to shoot for wringing as much clean headroom as possible out of a 2x6l6 design. I've had experience in the past with the UL Bassman Ten, and they were nothing but loud and clean, and since I already have the UL output transformer...that seemed a logical direction to go. The Triode input is 4.3k, stock later Bassman OT would be 4.2k. Close enough, I decided*. Yes the Triode is overkill, but it won't distort and boy will it be full frequency.

(And just to throw it in- I know those 2x6l6 UL amps claiming 70 watts...weren't.)

I'm shooting for simple- one channel, TMB tonestack, basic AB763 preamp circuit- married to the UL power section. As it is, I was following the idea of the "one tube" channel of a Bassman, so the circuit would be half triode- tonestack- half triode- phase inverter- power tubes. In tubes that means V1 12ax7 (maybe a 6sl7 since I have them) V2 12at7 and of course V3&4 6l6GC.

Absolutely nothing earth shattering or original, right? Haha. Simple, that's what I want.

Other than any general thoughts or feedback- which is always welcome- I only really have the following "questions":

- Power Supply Filtering: The real UL amps were running over 500v B+. I'm using a regular Bassman PT so will have more standard voltages. But interpreting the UL power section gets weird fast, since it looks to me like they had to incorporate a voltage divider after the first cap to get things under control. AND, I of course can't just go to the usual Fender filtering schemes since using the UL OT means I don't need a choke.

The schematic I am attaching is just a quick photoshop cut and paste of the Hoffman AB763 documents that I then started scribbling notes on- so none of the values are set. I'm surrounded by Macs so can't use the Duncan PSU calculator so fully expecting the dropping resistors to take a little trial and error...but I'm not wed to anything as drawn. Any thoughts are great.

Bias Circuit: This again is a bit weird in the Bassman Ten- obviously meant to be well filtered and then going into the bias balance. My schematic shows my attempt at turning the "stock" circuit into a more straightforward, adjustable circuit. Thoughts here are welcome, as it is again an area I have marked for trial and error.

I'd like to keep the 10k pot, since I have a couple on hand...but should I ditch the attempt to reinterpret the stock circuit and use something closer to the Hoffman "better bias" circuit?

At the end of the day this is a pretty use-specific amp, so I fully plan to be tweaking the "voice" a bit. The "best" Bassman Ten I ever heard actually had adjustable NFB, which loosened up the feel a bit- so I'll play with that. I included a plain-jane Type 3 Master Volume purely because MV was a specific request. This isn't for a guitar player so the usual "goose the preamp turn down the master" idea doesn't apply at all- and it certainly doesn't warrant a LarMar- I intend to use a push/pull pot for the MV so it can just be "off", but there if needed/wanted. Easier than arguing, let the ears judge.

I'll attach "my" schematic, the stock Bassman Ten, and a version of a Bassman Ten that someone modded to be more "guitar friendly". I came across the last somewhere online and was just using it as another reference point.

Thanks for reading, and thoughts appreciated!

[SILVERGUN]

This isn't for a guitar player so .....

What will it be used for?

[PRR]

The ".01?" really can be 0.001uFd. That 1Meg resistor is significantly bootstrapped from the cathodes.

If the client can afford it up front, I sure would load it with 6550/KT88. More reliable.

That OT was used by Sunn and one of their early amps was just about this simple.
http://el34world.com/charts/Schematics/files/sunn/sunn_spectrum_ii.pdf
You can always do worse than steal a Sunn design. Skip the vacuum rectifier and pi filter-- 1967 is come and gone.

The C-D drop resistor can surely be more than 1K. There's not even 2mA flowing in it. The B-C resistor likewise can't have 4mA so could be much bigger than 5K. You do not need Duncan to pencil-guess these values. I suspect the A-B resistor has 7mA, and can be run at 60V drop, so 10K here, 15K-22K B-C and C-D.

[92Volts]

Which power transformer are you using? I found a 355v one marketed as a Bassman replacement, which "should" give nearly 500v DC. Also, have you bought the PT yet or could you choose a different one?

4.3k ohms sounds low for 6L6GC near 500v, maybe EL34 could do better. Check out this article: http://www.valvewizard.co.uk/pp.html

Using the approximation at the end of that page, you could expect nearly 95 watts... "assuming the load impedance isn't unusually small for the type of valve being used". If it is unusually small, the tube won't be able to draw the peak current to realize that power output (and you may also exceed its dissipation limits).

This EL34 datasheet does give examples with 4k and 5k loads near 500v: http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL34.pdf producing 70 and 58 watts. Those examples aren't ultralinear but they show you're in the right ballpark. 6L6GC datasheets recommend larger loads.

I'd agree KT88 might be a good move here. Other than the expense of the tubes themselves, I don't think they'd require major or expensive changes to the circuit. I guess the main concern is whether your PT can handle the heater current?

[DenaliJoe]

The ".01?" really can be 0.001uFd. That 1Meg resistor is significantly bootstrapped from the cathodes.

If the client can afford it up front, I sure would load it with 6550/KT88. More reliable.

That OT was used by Sunn and one of their early amps was just about this simple.
http://el34world.com/charts/Schematics/files/sunn/sunn_spectrum_ii.pdf
You can always do worse than steal a Sunn design. Skip the vacuum rectifier and pi filter-- 1967 is come and gone.

The C-D drop resistor can surely be more than 1K. There's not even 2mA flowing in it. The B-C resistor likewise can't have 4mA so could be much bigger than 5K. You do not need Duncan to pencil-guess these values. I suspect the A-B resistor has 7mA, and can be run at 60V drop, so 10K here, 15K-22K B-C and C-D.

Hi and thanks!

Agreed on the bigger tubes- I had 6l6's on hand- and much of everything else, really- which is part of what drove the final decision to give this a shot. I figured if it's not "enough" I can go back and make the changes needed to bump up to the bigger tubes. The Sunn is a good reference, thanks for that.

Thanks for the that on the dropping resistors too, helpful.

I have a .001 I'll try first.

[DenaliJoe]

Which power transformer are you using? I found a 355v one marketed as a Bassman replacement, which "should" give nearly 500v DC. Also, have you bought the PT yet or could you choose a different one?

4.3k ohms sounds low for 6L6GC near 500v, maybe EL34 could do better. Check out this article: http://www.valvewizard.co.uk/pp.html

Using the approximation at the end of that page, you could expect nearly 95 watts... "assuming the load impedance isn't unusually small for the type of valve being used". If it is unusually small, the tube won't be able to draw the peak current to realize that power output (and you may also exceed its dissipation limits).

This EL34 datasheet does give examples with 4k and 5k loads near 500v: http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL34.pdf producing 70 and 58 watts. Those examples aren't ultralinear but they show you're in the right ballpark. 6L6GC datasheets recommend larger loads.

I'd agree KT88 might be a good move here. Other than the expense of the tubes themselves, I don't think they'd require major or expensive changes to the circuit. I guess the main concern is whether your PT can handle the heater current?

Hi, thanks- the PT I grabbed is a ClassicTone 40-18005, 330v...so it will stay under the 500v of the original and be more of the "regular" voltages of those later silverface amps. I was looking at the AB165 as a voltage reference which has 425v and figure I can come in above that, but veer towards similar preamp voltages as the Bassman Ten...

I need to fall back and check what a 6sl7 can take though, to see if it's possible to use one.

The PT has 4A of heater current. I think I veered this way not only due to the parts on hand but also simply knowing the circuit "works", since it's an old Fender design and I'm just downsizing it some...but it's within the "weird enough to be a bit of a stretch but not completely foreign territory" range for me. Totally open to the "don't be a knucklehead" argument, however, haha!

[DenaliJoe]

This isn't for a guitar player so .....

What will it be used for?

Yeah I hinted that it's not for guitar but that just set up a reveal that didn't happen...sorry...

It was a wee intentional since it's unique enough to pull all the focus.

The player is an electric violinist playing in what I guess could be called a goth-post-punk kind of band, they gig and record and are looking at more road dates next year, so the need for some reliable and portable gear is becoming more real. To date she's never had a rig that gave both volume and a nice sound live, but I've been working with her for a couple of months tuning up her rig. She uses both an electric and a piezo-equipped acoustic. Neither of them have preamps, so I started by building her a tube buffer like I use for acoustic guitar players. I then built her a new pedalboard with a clean power supply. All of that made such a difference the drummer even commented on it- so she's motivated. 

Next is an amp. She uses a lot of effects and is actually the lead instrument, the guitar player sings/plays rhythm. He uses a Twin- needlessly, he doesn't get above 5 even with no PA in a large venue- but he is an effects player as well so likes the clean headroom.

She's always used solid state keyboard or bass amps, the usual go-to in the electric violin world. I loaned her a Deluxe and she actually hung in there OK with it live, and sounded SO much better...

So that's the story. I found her a Mojo 2x12 Bassman cab that is now loaded with a Legend and an Eminence Big Ben 15", so I'm working the efficient crazy loud speaker angle too.

You guys are all validating my first impulse of going the 2xKT88 route, which is heartening and also making me second guess everything, haha. I think I landed on "build it and see what happens" since I had enough parts on hand to get going...rightly or wrongly...

[92Volts]

The 6SL7 would likely be fine. 12AX7s are used with 400+ supply voltages all the time (also rated for 300v). Preamp stage plate resistors eat about 1/2 the voltage so the plate voltage only gets that high before the tube is warmed up, or large signal peaks where it's not conducting current.

I'm currently using a 6SN7 (not the same tube, but also an older preamp tube) as a phase inverter with 600v B+ in a hifi amp. No trouble to report yet.

[PRR]

> I loaned her a Deluxe and she actually hung in there OK with it live

So build a big DeLuxe. Fender-stock all the way to the output tubes. Change those from 6V6 @ 8K to 6L6 @ 4K, use 2*6L6 OT and PT. Steal that whole last bit from any not-too-new (no UL!) Fender 2*6L6 design.

I'm a traditionalist. I liked cello through a Twin. It was maybe over-kill for the gigs that guy did, but it played jazz-clean until he really hit the bow, and this was the days that Twins were as common as classic Ford Galaxies (not that lump sold now under the name). Violin does not need the full bottom that a cello does, and if the DeLuxe is known-good I'd go that way, only upping the power for punk/goth play.