53-tube homebrew amp from 1992 - "Big Bertha"

[BigBandsMan]

I decided I wanted the folks on the Forum to see this brute, even if the pictures are not the best. I am offering it to encourage homebrew and custom construction of all sorts.





I built the first version of this unit in 1976. The basic idea was a high-fidelity 100-watt "keyboard amplifier" such as the commercial companies began building solid-state in the 1980's (such as the Peavey KB-300).

I took it through two rebuilds before ending up with this 1992 version, which I've stuck with. It has a number of special features, including a two-inch scope for diagnostics.

It weighs a ton, as you can imagine, and I can't move it around by myself too conveniently these days, so it generally stays here in my studio where I find plenty of work for it.

Many thanks for viewing, and hope this provides some inspiration. , Larry
_________________
Glenn Miller forever!

[35Z5]

WOW some piece of equipment... The thought of building such a piece boggles my mind...

Tom

[Mr. Detrola]

That took a lot of effort! And very nice looking too! Bet it took forever to design.

I built a lot of amplifiers back in the 60's and 70's, only have about 2 or 3 of them still around. Mostly copies of commercial designs, and nothing with more than a dozen or so tubes.
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Dennis

[BigBandsMan]

Thanks for the kind words, fellows. Yes, it was a hefty project, and I wouldn't even try to figure up the labor hours, but it was worth every one. There is always a reward in the end product, but there is a far greater reward in the journey.

Kudos to everybody who still likes to build their own gear. Keep it up!

All the best, Larry
_________________
Glenn Miller forever!

[War Bird Radio]

So where's the regeneration control?....

[BigBandsMan]

Good shot, WarBird!

Actually, I did build a little regen AM radio subchassis for this brute in 1993, but there wasn't room enough to mount it inside the case, so it sits on a shelf next door (out of view to the right). One of the cables in the harness leads to it. I don't have a picture of it yet, but it's a two-step TRF (two 6BA6) job, with a 12AT7 detector/AF out (cathode follower). Since I didn't have any bona-fide RF transformers at the time I built it, I jury-rigged Miller 71-OSC coils between stages. The result is something that tunes a little beyond either end of the standard broadcast band, but it's stable enough.



All the best, Larry
_________________
Glenn Miller forever!

[Big Nick]

Incredible! I've never seen anything like it and I've seen alot of equipment over the years.

So, may I ask the obvious question; what do the 53 tubes do?
A good handful must serve the scope.

And this is a keyboard amp? So, I'd assume it is intended for live/stage application and probably freatures multiple channels/mixer and what? Compressor, equalizer, etc.?

Where does the unit reside now, is it still being used?

Incredible!

--Big Nick

[BigBandsMan]

Thank you, Nick. I'll give you a rundown on the tubes and such.

POWER SUPPLY CHASSIS (lower deck w/small subchassis to either side):

2 - 5AR4, B+ supply #1 (output & driver stages), rectifiers
1 - 2D21, B+ supply #1 auto-standby trigger (fail-safe circuit in event of bias loss; disconnects output tube screens via relay when bias voltage falls below predetermined threshold; tubes will not conduct w/screens disconnected)
1 - 0A2, VR stabilizer for auto-standby
1 - 5AR4, B+ supply #2 (power amp chassis small signal stages), rectifier
1 - 6L6GC, B+ supply #2 , losser tube for VR circuit
1 - 0C3, B+ supply #2 VR circuit
1 - 6SJ7, B+ supply #2, control tube for VR circuit
1 - 6AF6G, magic eye, twin-shadow, B+ supply #2 regulated voltage level indicator
1 - 80, B+ supply #3 (auxiliary front end chassis), rectifier
3 - 0B2, B+ supply #3 regulator stack
1 - 6AL5, bias rectifier


SERVICE CHASSIS (mounted above power supply)

2 - 2X2/879, scope HV rectifiers
2 - 6X4, scope LV rectifiers (had to use transformer with low B+ output, so these are in voltage-doubler arrangement; worked fine, so never revised)
3 - 0A2, scope LV regulator stack
1 - 884, scope sweep oscillator
1 - 6SJ7, scope horizontal amplifier
1 - 6SJ7, scope vertical amplifier
1 - 7F8, scope retrace blanking amplifier
1 - 6AL5, clamper
1 - 2BP1, scope CRT
1 - 5823, mains ground status indicator (indicates open/nonfunctional mains ground)


POWER AMPLIFIER CHASSIS (second deck w/subchassis to left):

2 - 6SU7GTY, Channels A & B 1st & 2nd AFs (w/preamp bass-treble controls)
1 - 7F8, Channel C 1st & 2nd (cathode-follower) AF (with no bass/treble control)
1 - 6SU7GTY, 3rd and 4th AFs (w/all master controls)
1 - 6SF5 (5th AF)
1 - 6N7 (driver)
4 - 6L6GC or 7027A, push-pull parallel outputs (w/switch to change socket connections so that 6CA7/EL34 tubes may be used)
1 - 6SL7WGT (reverb driver)
1 - 6SC7 (reverb return)
1 - 6SL7WGT (tremolo oscillator & discharge stage)
1 - 6AF6G (magic eye, twin-shadow, output tube bias balance indicator)
2 - 6BN4 (control tubes for bias balance indicator)

AUXILIARY FRONT END CHASSIS (mounted above power amp):

1 - 6SU7GTY, Channels A & B 1st AF
1 - 6SU7GTY, Channel C (clip-filt) 1st AF & clip-filt driver
1 - 6SL7GT, Channel C clipper-filter
1 - 6SJ7, 2nd AF
1 - 6J5, cathode-follower output to power amp chassis
1 - 6SQ7, preamp for low-impedence headphone driver
1 - 6J5, headphone driver
1 - 12SL7GT, 30-hz & 1000-hz phase-shift test oscillators, generating sine waves for diagnostics
1 - 6E5, magic eye, output level indicator

In addition to the normal line fuse, there is at least one HV fuse in every B+ supply line.

Yes, it is a keyboard amp, and KBA's are high fidelity units. Since modern keyboards produce a very broad frequency spectrum, the KBA must be a high-fidelity unit in order to do its job right.

There are a number of experimental circuits mixed up in this. The auto-standby feature is the most important one. The clip-filt channel can do weird things for any instrument switched through it. The scope usually rides the amplifier output on internal sweep and sync, but it can be switched to external inputs, sweep and sync, and may be then used as any ordinary shop scope.

There is no compressor or expander. On stage, I have to work through a wide dynamic range, and I try to strike as natural a range as I can. I do use an outboard equalizer, but I always used that, regardless of the amplifier I was using.

The unit does very well on stage, but now that good commercial units (that weigh a lot less) are available, I seldom have to take it out anymore. In the studio, I use it as a monitor, and I get a lot of use out of the scope, mostly in aligning heads on tape decks.

All the best, Larry
_________________
Glenn Miller forever!

[akent36]

Didn't you used to work for Midwest?

[Curt Reed]

Actually it sounds like some of the transmitters I designed, but never got around to actually building them. VFO followed by a cathode follower for isolation, feeding a buffer, then a string of doublers that were switch selectable depending on the band, then a driver stage, followed by an intermediate higher powered driver and the finals in push-pull.

Then we turn to the audio stages for the modulator. Speech compression and the whole ball of wax. It looked good on paper, but in the real world, it would have been quite a monstorosity.
Curt
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Curt, N7AH
(Connoisseur of the cold 807) CW forever!

[BigBandsMan]

Hi, AKent 36. I'm afraid I never worked for Midwest.

Curt, perhaps you should go ahead and build one of those designs. Monstrosity or otherwise, who knows what fun it might turn out to be?

All the best, Larry
_________________
Glenn Miller forever!

[BigBandsMan]

I updated this thread and (many thanks Alan) had it moved to this forum from Homebrew Radios, since several other chaps here are involved in building their own amplifiers.

I hope they are as successful and happy with theirs as I was (and am) with this one.

Firebottles Forever!!!

Larry
_________________
Glenn Miller forever!

[Tube Radio]

Nice amp you have there!

[BigBandsMan]

Many thanks, TR. I only hope it demonstrates what can be done with vision and willpower (or sheer bull-headedness maybe).

I understand you do quite a bit of building yourself. Keep up the good work ,

Larry
_________________
Glenn Miller forever!

[Clay Nicolsen]

Amazing! What's the output power?

[BigBandsMan]

Thank you, Clay. It was built for 100 watts RMS. The power amp has a Fender Bassman 100 output transformer and was meant to work with 4 6L6GC's.

It gets along nicely with 7027A's too, which I prefer, because of their tone. EL37's also work well, but are a little too much on the boomy side for me.

6CA7/EL34's in that circuit are VERY boomy, but they can be used in a pinch.

All the best , Larry
_________________
Glenn Miller forever!

[Clay Nicolsen]

Sweet! I assume it's mono, and not stereo?

[marko]

How much juice does that thing suck idling?

looks nice

[BigBandsMan]

Hi, Clay. Yes, it's only a mono unit. I couldn't have moved around a stereo version. I could barely move this one at my best, and I'm a few years past that now .

Hi, Marko. It draws about 2.5 amps idling (mains fuse is 4 amps slo-blo). I throttle it as low as possible by biasing the output tubes deep into the AB1 range.

I appreciate the interest, guys. All the best to you,

Larry
_________________
Glenn Miller forever!

[Tube Radio]

Yes I do build stuff Larry, but most of my work these days is rebuilding things to make them work again.