Questions about using EL34, KT66, KT77 in Bassman 5f6a build

[lenniepickit]

I'm a bit of a newbie to this forum but I saw this great clip on a thread here while trying to search for EL34 conversion tips..':D'

http://www.youtube.com/watch?v=7bk4zeB3IrI

I've got bits of info here and there but can't seem to find a concise description of what needs to be done.
So what I like to know is what are the mods needed and which of those tubes will work...

Caveat here is that this is a 5f6a build in a Hotrod 1x12 cab that has an OLD Mojo output transformer in it w/2/4/8ohms that I've contacted the company about but can't find specs on online. ':roll:'
Model is MOJO 26L60 9651 and it was traded out from stock by the guy that helped me build into the non-stock cab.

Thanks!

[rdjones]

I'm a bit of a newbie to this forum but I saw this great clip on a thread here while trying to search for EL34 conversion tips..':D'

http://www.youtube.com/watch?v=7bk4zeB3IrI

I've got bits of info here and there but can't seem to find a concise description of what needs to be done.
So what I like to know is what are the mods needed and which of those tubes will work...

Caveat here is that this is a 5f6a build in a Hotrod 1x12 cab that has an OLD Mojo output transformer in it w/2/4/8ohms that I've contacted the company about but can't find specs on online. ':roll:'
Model is MOJO 26L60 9651 and it was traded out from stock by the guy that helped me build into the non-stock cab.

Thanks!

Wire the output tube sockets for "universal" 6L6/EL34 use.
Ground pin1. It can be grounded directly to the chassis. It doesn't carry any current, it's just a ground reference.
Ground pin 8. This carries all the output tubes current and should go to the junction of the B+ filters negative and the power supply negative.

This will allow the use of all the 6L6 and EL34 variants, including KTxx's and 6550s.

How closely are you following the 5F6A circuit ?
edit: I just verified that the original layout for the 5F6A has the proper pins grounded already. I guess it pays to check things first
The original circuit has screen grid resistors but no (control) grid stoppers.
If you add the stoppers you will need to use pin 6 as the tie point for the grid stoppers and relocate the screen resistor. (the grid stoppers, if used, need to be right on the control grid terminal)
You'll also want to have an adjustable bias with enough range to handle the variety of tube types, and increase the screen resistors to 1k.

rd

[martin manning]

The original circuit has screen grid resistors but no (control) grid stoppers.
If you add the stoppers you will need to use pin 6 as the tie point for the grid stoppers and relocate the screen resistor. (the grid stoppers, if used, need to be right on the control grid terminal)

...unless you do the grid stoppers Marshall style, with the resistor soldered close to pin 5 and the grid lead soldered directly to the other (very short) lead.

[lenniepickit]

Thanks for the replies - I like the idea of being able to switch out tubes there...

Yes - pins 1 and 8 are grounded to the chassis, so let me summarize the two posts here and correct me if I'm wrong so I don't blow something up lol...

1) Increase grid resistors across pins 4 and 6 to 1k2w

2) Run new grid stoppers (1k - 5k 1/2w?) directly off pin 5 to my bias resistors


BTW I talked to one of the guys from Mojo and he said that OT was from back when they used the same tranny for both Bassman and JTM kits and that it's over spec'ed and should be fine..

Don't worry - I won't be starting this till it's all clear

[ToneMerc]

You will need to modify the bias supply system as well.

TM

[lenniepickit]

I just noticed a nice spot for trim pot mount yesterday - there's already a hole drilled in the chassis for it I think..

So then if I get those resistors in place and ready to reset the bias then I'd be set to pop a set of EL84's in?
Or kt66/77's?

Thanks for the great info!

[Reeltarded]

Not 84s.

(Yeah, I know fast typer..)

[lenniepickit]

Not 84s.

(Yeah, I know fast typer..)

Lol - yep - though I will admit it was the EL34's that got me in trouble with the whole tweaking thing in the first place : )

[Reeltarded]

Forward throat. Best tube for everything. I even keep toothpaste in 'em.

lol

[lenniepickit]

Hey ya'll,

Well, I got the 1K resistors in place across pins 4 and 6 and pins 1 and 8 are crossed and grounded to chassis. I skipped the grid stopper as the run was long it seems many of the old amps don't have it.

Then I decided to pop the EL34's in to see things were in the ballpark before adding the bias pot since the JTM schematic has the same bias resistor as the 5F6A. But the amp sounds very quacky and farty and the tubes look like they are burning hot..

Will post pics later if there is no reply but while I run the fam out on errands I was wondering if there was anything obvious that I might have missed - it seems unlikely the bias would cause that much sound degradation but I've never tried a Power Tube set up swap either...

Thanks!

[ToneMerc]

Hey ya'll,

Well, I got the 1K resistors in place across pins 4 and 6 and pins 1 and 8 are crossed and grounded to chassis. I skipped the grid stopper as the run was long it seems many of the old amps don't have it.

Then I decided to pop the EL34's in to see things were in the ballpark before adding the bias pot since the JTM schematic has the same bias resistor as the 5F6A. But the amp sounds very quacky and farty and the tubes look like they are burning hot..

Will post pics later if there is no reply but while I run the fam out on errands I was wondering if there was anything obvious that I might have missed - it seems unlikely the bias would cause that much sound degradation but I've never tried a Power Tube set up swap either...

Thanks!

Yes, the most obvious modification, you need to modify the bias supply. All you did was increase screen grid resistors and modify the sockets for EL34's
The JTM45 like the Bassman did not use the EL34. Install the pot, hopefuly you didn't cook those power tubes.

TM

[lenniepickit]

Would be wrong to say that I was simply trying to follow - oh no - yes, I have to...
the path of least resistance


Sorry - couldn't - doh - resist - honest I didn't mean that one but I did put the 50k linear in the circuit and have the tubes tweaked and the rig is sounding sweet as honey!

The break up is sooo much smoother with the EL34's - yes there is a bit less low end than the 5881's but the chime and harmonics are outa this world..

So 2 more questions..

Once I got that pot tweaked the reading was surprisingly just about 14.7k - is there any reason I can't just hook that 15k resistor up if I'm going to roll with these changes?

Also, it seems the EL34's run a bit hotter 5881 - I think my plate voltage is about 430 so I think that is fine so I'm assuming it's just a tube characteristic like the little EL84's in my Mesa..

Thanks again folks!