Bass Amp for a friend

[jbrew73]

I only used 6sl7 (vintage used) in one low watt project and had trouble with microphonics. You might want to consider another tube for a high wattage bass amp. Maybe new tubes would be fine though.

[Colossal]

[/quote]

Here's some old Dynaco transformer info you may find interesting. It covers the 451. Pretty expensive back in the day.

Steve, thanks, that was helpful. Man, I wish that A451 was still $59.95!

This is the thread I was talking about:
https://music-electronics-forum.com/sho ... hp?t=28617

Thanks Roberto, this was a great read. I got some good ideas and some perspective from it.

I only used 6sl7 (vintage used) in one low watt project and had trouble with microphonics. You might want to consider another tube for a high wattage bass amp. Maybe new tubes would be fine though.

Thanks. That thought had crossed my mind and I don't really want to add a DC supply to keep them quiet. If used, they would have been new production Tung Sol. I was thinking along the lines of the B-15 preamp with the 6SL7s. But certainly 12As would probably be easier.

[Colossal]

I emailed the guys at Heyboer for a quotation for PT, choke, and OT.

[TUBEDUDE]

D.C.won't help microphonics anyway. Elastomer washers under the tube socket mount, and a sorbothane isolated sub chassis will help if you have to use microphonic tubes.

[M Fowler]

The one bass amp I built from scratch for a friend, a local blues bass player: Preamp design borrowed from Fender's white/tan early 60's Bassman, bass channel of course. Power amp 4 x KT88 or 6550 developing 120-140W thru a Hammond 1650T OT. Power transformer Hammond 378CX 400-0-400 V 534 mA. Way over spec'd for the application, it doesn't sag. Simple design, uncompromising power.

I built nearly the same amp, Fender Bassman 100 preamp, same PT and OT as Leo stated.
First I ran 4-KT88's but the screen voltage was too high I figured for KT88's and the guy needed back right away so I switched it to 4-6550's and the amp is gigged every week Bass guitar and hasn't been back since and I know it's being gigged I get the band photos. Plenty of volume and these kids are freaking loud.

Mark

[M Fowler]

I also built a 600w Class D power amp into old Altec-Lansing power amp control panel and new chassis. This amp has balls.
Tube preamp I built goes in front.

I also built the Weber SO100 for my brother in law for bass use and it is a fun amp but Dave's friend needs more wattage.

Mark

Or the BFA we have talked about before Dave.

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BFA_front.JPG

[dorrisant]

Mark, was that a drop in power amp for the Class D amp? If so, where from?

Also, dare I ask what "BFA" stands for?

Inquiring minds want to know...

[M Fowler]

I first used a Shure 600w Class D power amp module with a Meanwell 48v power supply.

I have yet to turn that 600w amp to full power it's still on the lowest setting.

Currently I'm using Icepower amp/power modules in one unit nice and small that you can buy from Parts Express.

Working on 250w in Champ combo cabinet taller to fit Milkman 12" speaker.

Big Fucking Amp!

[dorrisant]

Right on Mark!! You rock!

[Colossal]

Ok, a quick update on this project.

I've been spec'ing transformers with Phil from Heyboer. OT will be a Dynaclone A451 at 2k2. Power supply will be SS full wave rectified into a CLC filter. Then B+ to OT. Choke will be rated for 500mA.

I will be running a quad of Winged C 6550s. Max dissipation is 35W. The datasheet specifies the U/L application at either a plate voltage of 560V for 100W or 450V at 70W. Any thoughts on the operating point? (Ian, Tony, Leo, et al)?

For the PT, I am specifying either 400VAC or 410VAC at 450-500mA secondary, a 55VAC bias tap, and two 6.3VAC taps; one for heaters (9A) and the other for accessory stuff (3-4A).

For my bar napkin math using a 400VAC secondary...

With a 400VAC secondary, 400 * 1.3 = 520VDC rectified under load
520VDC across a CLC = ~ 10VDC drop
510VDC B+ to OT centertap
~ 500-500VDC plates
~ 224VDC screens @ 43% screen taps
~ -60VDC bias

[dorrisant]

Sounds devine Dave. Are those off the shelf or special order?

[Colossal]

Sounds devine Dave. Are those off the shelf or special order?

Hi Tony, they will be special order. Phil said they used to make and stock A451 but not anymore. PT will be a custom wind to get the current handling up.

I just wanted to get some input from you guys regarding the PT and a target operating point. I agree with Ian and Leo on going with pretty high voltages as well as a well filtered supply. That 140W output is based on a quad running 460V plates at -59V bias. Going up to that 560V plate and the bias goes to -75V. I was thinking of landing in that 490-520V ballpark. I am using a quad of 6550 pulls so don't want to beat on them unnecessarily, just because. Also, I want to ensure that future retubes are easy with no arcing or other problems.

Dave

[tubeswell]

For the PT, I am specifying either 400VAC or 410VAC at 450-500mA secondary, a 55VAC bias tap, and two 6.3VAC taps; one for heaters (9A) and the other for accessory stuff (3-4A).

For my bar napkin math using a 400VAC secondary...

With a 400VAC secondary, 400 * 1.3 = 520VDC rectified under load
520VDC across a CLC = ~ 10VDC drop
510VDC B+ to OT centertap
~ 500-500VDC plates
~ 224VDC screens @ 43% screen taps
~ -60VDC bias

I see you are going UL (so that rules out using a separate screen supply).

But you might want to consider a bias supply that delivers a highish -ve VDC for running 12AU7 CF buffers that directly feed the 6550 grids. This will keep the 6550 grids happy. See Ampeg SVT attached

[Colossal]

I see you are going UL (so that rules out using a separate screen supply).

But you might want to consider a bias supply that delivers a highish -ve VDC for running 12AU7 CF buffers that directly feed the 6550 grids. This will keep the 6550 grids happy. See Ampeg SVT attached

Well, I had considered it, and still might if you guys think it would be beneficial to the design. They make a difference in the authority of the bass in Steel String Singers. I thought it would be fun to collectively discuss and decide on the topology for this amp so nothing is set in stone yet. What do you think of this CF assisted cathodyne for the PI/Driver?

Cathodyne.jpg

[tubeswell]

Yeah that will also work.

But in the SVT, the ‘fixed voltage’-end of the CF load resistor is referenced to a really negative voltage, so that the 12AU7 cathodes can be DC coupled directly to the output tube grids (to eliminate blocking distortion). The CF load resistance doubles as the output tubes’ grid leak path. The bias control adjusts the reference voltage of the 12AU7 cathodes (which determines the 6550 bias voltage).