Tube bass amp...

[Bob M.]

I've been commissioned to design and build a tube bass amp. Initially, I though I would use 6550 power tubes but I'm thinking about the newer KT120 and/or KT150 tubes. Does anyone have any experience building using these new power tubes and if so, could you please elaborate on your findings?

Thanks in adv,

Bob M.

[Jazz P Bass]

While I admire you for accepting the job, I feel that ANY tube amp is not going to cut it in a live band.

Solid state is the way to go.

There are some awesome SS amps out there with power to drive & preamp controls to make you drool.

Just my 2 pence.

[J M Fahey]

Can't comment on tubes themselves, but my doubt is: what kind of iron will you use?

*If* you use, say, popular Hammond transformers as used by Ampeg/Sunn/Hiwatt cloners, simply because of availability, choosing newer improved tubes instead of classic KT88/6550 won't really change the game, because plate voltage swing will be basically the same (since you will have same +B) and current swing again basically the same, because you keep load impedance.

Now if you use custom iron, then the improved tubes may yield higher power, no doubt.

Personally I wouldn't try to fully reinvent the wheel, specially for a one-off ; I'd base my design on some time tested backbone and add as many extra details and conveniences as to make customer happy, specially at preamp level (effects loop / graphic or parametric equalizer / tuner out / switchable channels / etc.)

Of course, if you go over the top, larger tubes mean less of them so they simplify construction.

Here Mr Chambonino went from 20 807 tubes:

notice the row of fans

to "just" 4 813 in his second 1000W amp :


custom iron of course, and he needed a separate chassis for the power supply

compare that to SS 1400W RMS just mentioned in another thread:

[GainFreak]

Here Mr Chambonino went from 20 807 tubes:

Mr Chambonino definitely went the extra mile

[TimmyP1955]

Tell him to buy an SVT head (which are still on a LOT of riders), then fix the EQ to his liking.

[J M Fahey]

Tell him to buy an SVT head (which are still on a LOT of riders), then fix the EQ to his liking.

You not a builder I guess

The service is to oneself first, and the customer in second place.

Custom building a head can fetch between $2000 and $8000 , we don't know the circumstances here, and may bring further work of the same quality ($$$$$) ; while it will be hard to pull more than $100 to $200 from a Musician's wallet just for "tweaking" , even more if the original amp is considered quite good as is.

Simple Musician-Tech marketing reality.

In a way, similar difference between writing song and music for a Pepsi commercial versus playing a Beatles song cover , *sligtly* modded to add the "pepsi" word.

[Bob M.]

Thanks for the comments. Yes, I don't really intend to reinvent the wheel here. I will probably base my design on something well proven in the past, with my own ideas interjected about preamp, PI, speaker(s), whatever. And yes, this guy has a closetful of good solid state bass amps but would like a tube bass amp for smaller gigs and recordings. Remember, there are all types of bands out there these days and some of them actually play great soft (not loud) music. It's like having a different type of bass guitar for a different sound or flavor in his arsenal. I built him a guitar amp last year that he loves so he's asked me to do a tube bass amp. Money is not really the object here but it's more doing something really nice and unique that makes a nice, warm joyful (low end) noise (without the hum).

Of course, I realize that the transformers would integral to the design but I was hoping that someone on this forum had actually tried using these tube types and wondered what their experiences were. I build a fair amount of amps and have a pretty good track record but I haven't used the KT120 or KT 150 (yet) in a design. I like to keep learning new things - it keeps me young but I'm not too old to learn from others and there are many sharp people here on this forum to learn from.

Thanks,

Bob M.

P.S. Like the photos !

[jmaf]

5 years ago I set out to build a KT120-based guitar amp. My day job and other obligations began to take most my time and I never finished it.

But here's my 2 cents :

You'll need a low impedance bias supply. Can't have the traditional 220k resistors and a high impedance voltage divider to set the bias.

Grid to ground impedance should not exceed 50 KOhms for a KT120. KT120's go into thermal runaway easily if your bias isn't stiff enough to hold the grid down. So that's design challenge #1.

Challenge #2. You need a lower impedance driver to accommodate the lower input grid impedance. If you use the traditional Schmitt inverter you'll probably kill your tone and you probably won't be able to drive the KT's to full power. I tested with a Fender PIV and it sounded like crap.

Then I started experimenting with a 12AU7 with the 2 triodes in push-pull and had second plans for a singled ended EL84 + transformer based phase inverter. KT120's are beasts, the driver's where you'll need to focus on. The 6550-based Ampeg SVT's have a rugged driver as well, you could use that for a reference but I don't know about patent issues and I also wanted to go my own way so I didn't base off of Ampeg.

That's when the family and job got the best of me and I had to stop my KT120 adventure ("for now", but it's been 5 years). I was gonna use a preamp I designed in 2010, so I had that first part ready.

Hope this helps somewhat.

[mhuss]

Just use ECC81/12AT7 cathode follower drivers for the grids, as Hylight did for their 6xKT88 amp.
http://hiwatt.org/Schematics/DR_400wOutput.pdf
This keeps the impedance low and provides plenty of juice to keep the grid voltages in line.

[jmaf]

Just use ECC81/12AT7 cathode follower drivers for the grids, as Hylight did for their 6xKT88 amp.
http://hiwatt.org/Schematics/DR_400wOutput.pdf
This keeps the impedance low and provides plenty of juice to keep the grid voltages in line.

Have you tested that circuit with KT120's?

[mhuss]

No, I haven't experimented with those. However, three KT88 grids in parallel is a fairly robust load!

One other note, the -130v supply on the schematic is from a separate winding, full-wave bridge and filter, so can provide much higher current than a typical 'high resistance off the main HV winding' circuit. (Note the unusual resistor on the ground side.)
http://hiwatt.org/Schematics/DR_400wPS.pdf

[bea]

Just a possibly silly question: what about running KT88 at some large anode voltage, e.g. 780 V?

A pair of those should be capable of doing about 150 W under these conditions (extrapolated from the 100 W which are possible with 2 EL34). Custom transformers provided, of course, and possibly also an ECC81 as cathode follower to drive the grids. Maybe a replacement transformer for the power supply of the Dynacord Gigant, which is still available, could be used for 2 valves.

BTW: i am aware of at least one successful (amateur build by an electronics engineer) build of an amp obtaining around 280 W from 2 KT120. He drove the grids with two power MOSFETS.

[Justin Thomas]

Is there a BABA equivalent of the BAGA? Is kg still around? Anyway, he made a Hugh-Jass KT90 guitar amp...

Justin

[mhuss]

You could certainly run a pair of power tubes at 750v (EL34, KT88) as long as you have a separate screen supply about half that of the plate. In addition (and as you allude to), you'd need an appropriate OT with a primary impedance (much higher than that for a 500v amp) that would keep the plates in their safe operating area.

[bea]

...much higher than that for a 500v amp) that would keep the plates in their safe operating area.

Indeed.

For the KT88 i would expect something in the range 5-6.5 k primary, extrapolated from the 8-10 k used with the EL34 (again using the anode current ratio).

The main advantage of such constructions appears to me the larger efficiency due to a) less heating current current and b) smaller quiescent current.
And that offers the advantage of possibly smaller and more lightweight enclosures.