Bassman 10 mod--Plexi with 6L6GC

[chinrest]

Howdy,

I'm modding a Bassman 10. It's a 1972 50 watt (not UL).

Originally I thought I could just convert it to the 5F6-A circuit, but the PT voltage was quite a bit higher (365vac versus 325vac). Then I thought that I would convert it to a Marshall plexi and leave the diode rectifier.

Just for kicks, I modified the preamp to the Marshall specs, installed a choke (holes were conveniently pre-drilled in the chassis) but left the 6L6GC power. The 6L6GC's (RCA) are running at max plate voltage (500vdc), and I biased them at 21W dissipation.

I could just reconfigure for EL34's, but I wanted to see if anyone else had comments about a Plexi pre with 6L6GC output.

[imaradiostar]

How does it sound?

Depending on what you want out of it you might prefer the 6L6 outputs. Eric Johnson's dirty rhythm sound is a 50 watt plexi with 6L6's. Some accounts say that Hendrix favored 6L6;s over EL34's in his Marshalls.

Differences are less evident if you're using the amp at less than deafening levels.

jamie

[cbarrow7625]

I put a plexi circuit in my bassman 100 many years ago. I loved it. It definitely had a Hendrix vibe to it. I couldn'y resist playing "Voodoo Chile" every time I plugged in.

I've never been an EL34 fan so I'm biased, but the 6l6's really give a nice, round full sound that is much groovier & fuller than an EL34 with that circuit.

BTW, you may not be able to just dump EL34's in that amp. The heater current requirements of EL34's are much higher than a 6L6's. Most Fender amps don't have the heater current headroom for that swap. You don't want to burn up a power transformer!!!

See what you can dig up on that transformer before trying it. The best way to find out is to look at other Fender ams that use the same transformer & see if the total heater current draw for the amp with the most tubes is the same or less than the added current draw from the EL34's in your amp.

Chris

[chinrest]

Thanks guys,

I definitely like the sound. That's why I didn't follow through with the EL34. As far as the PT holding up, I was going to take a slightly-educated wager and say that it would hold up (while monitoring the temperature for possible meltdown).

The only thing that bugs me is the whopping screen voltage: 503vdc. It seems that the Bassman 100 PT produces around 450vdc, as opposed to the Bassman 10's 500vdc. The Bassman 10 uses a 2K2 resistor to drop the screens down to around 450v. I'm wondering if I should drop the screen voltage (resistor in series with choke?).

[Amp Kat]

Your screen voltage should be about 5 volts lower than the plate but I'm not sure you should add a resistor in series unless you want more sag at higher volumes as it may get musshy on you. If your going to use EL-34's then use 1k5 watters but 6L6's draw so little screen current it shouldn't be a problem and should take 500 easily with sufficient bias. That is actually a good modding amp as theres tons of room inside it. Hendrix used 6550's in all of his amps especially later and if he only would have gotten his hands on some 88's it would have been that as he was all about power tube distortion with as much power as he could get.

[chinrest]

Thanks for your comments.

I'm leaning toward going with EL34's--I'm a little worried about burning up the 6L6GC screens.

A very brief search turned up general specs for Fender 50watt power transformers: 700V / 140MA 6.3V / 4A 5V / 3A. A pair of EL34 and three 12ax7 adds up to 3.9A.

I'll see if I can find more info, part#, etc.

[chinrest]

500v on 6L6GC screens-maybe use KT66?

6L6's draw so little screen current it shouldn't be a problem and should take 500 easily with sufficient bias.

AmpKat, are you saying that the 6L6GC can take 500V on the screen grid? I haven't come across any designs that run the screens this hot, and the max rating is 450V. I'm willing to give it a go though.

What about KT66? The JJ brand purports to handle 550V on plates and screen. The heater current is a little less that EL34, so that would ease some of the worry about melting the PT. I've never used these tubes before.

[imaradiostar]

ooops- dragging up an old thread!

There is no reason not to use the choke and a dropping resistor if you like the way it sounds and it keeps the 6L6's happy! Are any elements in the tubes other than the cathode glowing?

Beam tubes draw much less screen current than a true pentode. You could always connect the 2k2 resistor like it used to be and add a second pair of 1n4007 diodes from the PT secondary into the choke and a cap just to run the screens, as suggested by Bruce in other threads. Given 365 volts AC at the secondary you'd end up with around 325 volts to run the screens. If you wanted to you could run the rest of the pre and PI from that supply and you'd probably have a different feel and different headroom in the preamp.

Does the amp have a tube rectifier? If not you might consider adding one with the little 5 volt 3 amp tf Weber sells. A 5u4 would give you a pretty healthy voltage drop if you wanted to pull the plate voltage down. If the amp has a bridge rectifier now you can replace the positive facing diodes with a tube rect and keep the SS diodes facing the ground side.

jamie

[chinrest]

Thanks Jamie,

The amp is running good and sounding great. I am curious about the second set of diodes. If you have a minute to explain that, or to direct me to another thread, I'd appreciate it.

Cheers,

Chris

[imaradiostar]

http://music-electronics-forum.com/s...ead.php?t=5620

Somewhere on page 2 Bruce suggests it- I've messed with it a little bit on the bench but haven't committed anything to a finished build. I have a few transformers that end up with too much plate voltage and rather than wasting them as heat in a zener or the like I'm going to adjust the circuit to be OK with the higher plate voltage.

Here's a quote:

Here's something you can try though to get lower screen voltages and high plate voltages.
Run a separate pair of 1N4007 diodes off the PT secondary windings into a simple full wave rectifier and feed the DC from the cathodes into a 100ma to 150ma, 5Hy to 10Hy choke, followed by a healthy sized, 47uF@450v E-cap.
This will be your dedicated screen supply... which will draw current from the PT under load but the DC voltage derived will not exceed about 90% of the actual Hi-Vac when run into the choke input filter and it's filter capacitor.
In your case this will be around 360vdc.

Now I would not even bother with the SS vs tube rectifier switching but, to add to your questions and desires....
in parallel with that new screen node circuit run your Hi-V secondary leads over to a good DPDT switch, wired so that the center lugs can steer the H-Vac to either a rectifier tube or another set of SS 1N4007 diodes in FW with a capacitor input filter for higher B+ and the rest of the B+ rail, ... which of course will be the high current B+ rail for the power tube's plates and the rest of the preamp section, if desired to take it from there.
(You could take the B+ for the preamp stages from the new screen node if you wanted as the preamp will not draw that much current either and that might be better since you will already have a lower B+ node to work with.)

jamie