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70.22.205.193
A friend of mine has one of these guys thats in desperate need
of cleanup. Yes, it works and he's even played it, but from the
looks of the electrolytic caps underneath - its a ticking timebomb.
It's a 90W bass amp, but we're going to downgrade it to guitar duty.The Power Trannie on this thing is UNBELIEVABLY HUGE!
What do you guys think about using KT90's or modern 6550's
at these volts? Before I start lion-taming the B+ supply I'd at
least like to see what you guys think about whats out there that
might run at the extremely hot and hard operationg point. I'm
skeptical of what might happen if he tries to put new Sovtek EL34s
in this with 530V on the screen.In my restorations efforts, besides the obvious coupling cap and
electrolytic replacements, I'd like to try to tame this beast so
that it doesnt just eat tubes for lunch. Obviously, I'm considering dropping down the B+ to maybe 500V by rebuilding the supply for slightly more kinder CLC operation (using no more than 8uF input-C likely a film/foil/oil Industrial Motor run cap to take the ripple). There's not much room for nor filament windings for tube rectifiers here. However, the transformer's huge and with diode rectification, this thing is just going gush current.I intend on keeping the original tone stack and Marshall driver but
makes some tiny changes in the "Range Extender" circuit for the
electric guitar.The screen grid supply is terrifying... It's just a 470 ohm shared
10W sand resistor between it and the PSU... If you were going to choose a screen grid R that's explosion worthy, what'd you be?
A Mills 10W ?-- Jim
Follow Ups:
Peter Traynor used a "trick" that allowed higher Vp and Vg2 voltages by tying the supressor grid to the raw negative bias supply. See if pin 1 ties to the bias supply - if not it's probably grounded or tied to the cathode (pin 8). Also he switched from Mullard EL34s to the USA/Canadian produced "big bottle" (like there ever was a "slim bottle") 6CA7 which is a much more robust tube than the EL34 (disassemble a few dead examples and note that the plate material is a couple of thousanths of an inch thicker and there is much more surface area).While I've seen some comment over the years that suggested that a negative supressor grid made for a "colder" sound I've not observed this personally. If the original 6CA7s work leave 'em in - the newer "issue" of these gets good reports but I still doubt that they are a rugged as the originals and 6550s definitely sound "cold/sterile" in these amps.
Rob
Those old Bassmasters are killers! I have a YBA-1 (not the 'a', that runs lower voltages) and I won't part with it. I sold off most of my guitar amp collection a few years ago, including a few that I really miss, but that's one of the ones that I won't sell.Anyway, one thing that I have done to lower the B+ in a couple of old amps is to add a big honking, stud mount zener between the power transformer center tap and ground. Some of the Traynors used a bridge rectifier, but the zener should still work between the bridge and ground.
I did a little search to figure out which part I used... Must have been from the NTE line that Allied offers (link to data sheet below.) You would want one with the cathode connected to the stud, so it would have a 'K' at the end. Allied doesn't stock them all, but they will get whatever you want pretty quickly. For some reason that I have not figured out (and haven't thought about in a log time) the drop in B+ is somewhat larger that the actual zener voltage. A 56V zener will drop B+ something like 65 or 75 volts, which should be about right.
Also, there were quite a few different Bassmasters. Some of the later ones had creative circuits, but most of the earlier ones (most of the ones I've seen) were actually pretty similar to early Marshall / tweed Bassman, meaning that the hi and low range extender controls were really just mid and presence controls. I think they actually sound pretty good the way they are, but don't let me stop you from experimenting.
One thing that they started leaving out was a choke in the power supply. The chassis's are huge, so there's plenty of room to replace the 470 ohm (or whatever) resistor that they started using instead. You don't need much, just a couple of henries at 100mA or so.
As for the single shared screen resistor, I like them. You could try replacing it with a separate resistor at each screen, but I think the common one contributes to the sound of these amps. 10W Mills should work very well either way. Could probably get away with 5W.
-- Dave
Hi Dave:
I have heard of that technique. Does that still work when the bias supply is derived from a tap on the HV winding, (like Fenders)?Does the zener also affect the bias supply?
Thanks in advance! Al
"Does the zener also affect the bias supply?"Yes it does! Thank you for waking me up!
If the bias supply is derived from a tap, then there likely will not be enough raw voltage left over, depending on how large a zener is used. You can always change it to use the entire transformer winding, though you might want to switch to a higher voltage diode. Obviously, a resistor or two will need to be changed as well.
If the bias supply is already derived from the entire winding there might be enough adjustment left to bias the tubes the way you want. OTOH it might be necessary to change the dropping resistor to bring it up a little.
Thanks again for bringing this up.
-- Dave
That would concern me with the el34. The new flying "c" tubes appear to be very well made. I wouldn't sweat the anode voltage. But I'd consider dropping the G2 voltage down around 400VDC (which should be cheap and easy to do).IIRC the 6550 has a lower voltage rating than the el34.
Russ
I had one of those come through my shop a while ago and the customer had problems with tube life. The solution was to build a seperate screen supply. Most of the common power tubes will work now.
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