Old swedish pa amp conversion

[5Trout]

Hello, found one old pa amp from the 60's.
I wish to convert it to a guitar(maybe also harp) amp.

Any input or design suggestions are welcome, since I only build
one tube kit amp before...

Someone has put 6l6GC tubes in it and the ecc83's are mesa 7025, but the
rectifier and ef86's are Mullard.
I don't plan to buy any new tubes for it, if something has to go it will be the ot
i suppose...

I've looked at some old fender cathode biased amps, because that's the sound
I'm want.
I also played with the thought to add a ef86 channel.

The ot is not optimal for a guitar amp I suppose(don't really have much
knowledge about the construction, really), but maybe it's usable?
It has 4,8,16 and 100 ohm taps.

Some voltages off the PT:
Line voltage sel.(vac) 240 220
heater(vac) 5.8 6.3
PT sec(vac) 345 375
B+(vdc) 425 462
measured with the tubes in.

Some pictures:

[Ray Ivers]

Two very simple/easy mods would be disconnecting R13, and replacing R39 with a 1M pot, with the wiper connected to R38.

Ray

[sportster4eva]

I've used some of those particular output tubes, and at those plate voltages you'll need to keep an eye on them I think. The plate construction is less robust than current production or the good old US made 6l6GC's. Being a cathode biased amp, they may be ok.
Just my $.02

[5Trout]

Thanks for the input, Ray and sportster4eva.

Ray, do You mean removing R13 and keep the connection to R45/46 from the
ot, or remove the connection completely?
What do the connection including R13(from ot to pi) do?
R39 controls the gain from the preamp, right?

What about the ultralinear ot, should it be a bad idea to disconnect R55/56 from
the ot, connect them both somewhere at the C23 cap, maybe adding a choke?


sportster4eva, I was a little suspicious about the 6L6's. They have been used
in the amp only for a few hours, maybe.I don't have a clue why someone have
switched tubes, but I think the 6l6's have replaced some other 6l6 that broke.
The voltage selector was set at 220v when I got the amp, so voltage was
plenty
Might be a good idea to change the cathode resistor, maybe 270 ohms in R53?

[Ray Ivers]

Ray, do You mean removing R13 and keep the connection to R45/46 from the
ot, or remove the connection completely?

I meant removing the connection completely.

What do the connection including R13 (from ot to pi) do?

This is negative feedback around the power amp.

R39 controls the gain from the preamp, right?

If R39 is replaced by a pot, I would say it's more of a level control to the power-amp section, allowing you to control power-amp drive independently of preamp drive.

What about the ultralinear ot, should it be a bad idea to disconnect R55/56 from
the ot, connect them both somewhere at the C23 cap, maybe adding a choke?

I would listen to the amp after doing the mods I suggested, and see how you like it. If it were my amp, I would definitely keep ultralinear operation, even if I changed the stock OT to another UL OT; to my ears it combines the best characteristics of triode and pentode operation.

Ray

[5Trout]

Thanks for clearifying, Ray.

I certainly try what You suggested, must change the input/output jacks
first and a grounded mains cable may be a good idea i guess...

/Magnus

[Turbo7MN]

Cool amp!

One thing I like doing in my PA conversions is putting a SPST cutoff switch in the NFB loop rather than removing it completely. It does require drilling a hole in the amp though.

Replacing R39 with a pot is a good idea. Never hurts to have a master volume, assuming you have space to put the pot.

I'd think of putting EL34s back in too.

One thing's for sure, get new jacks mounted and play around with it as-is before you start changing things. Get a feel for how it sounds now, then decide what else it needs. It's happened to me where I got a schematic before I played through the amp, and spent a bunch of time working out what I was going to change only to change my mind on a bunch of stuff after I played through it. Are you looking for a good clean sound, or overdrive?

By the way, do you have a larger copy of the schematic by any chance? Some of the component values are a bit hard to read.

Good luck on your project!

-Darren

[5Trout]

Turbo7MN, I want a good clean sound, but no harm if it breaks up
in a nice way There are pedals if overdrive is desired.

Have a better schematic in pdf form, but it's 1,4meg so posting it here was not
possible. Mail me and I will send it to You.

The amp I got differs a little from the schematic, it's a 3 channel amp.
It has 2 mic inputs and one turntable input.
The 2meg pots at the mic inputs are not present, and when I checked inside the
amp yesterday I found out the NFB loop looked like the "nfb.jpg" image atached.
It's like one more connection to ground from the ot that's not in the schematic.
Maybe not the same ot...

The IFB is located at the ot primarys and the OFB at the secondarys.

Maybe a good idea to draw a new schematic from this amp, but it's a bit
messy to follow the cables. The schematic for the five channel helps though.

[Turbo7MN]

If it's not exactly the same, there's no point in me having a bigger copy of the schematic .

If you think you can draw a new schematic, by all means go for it. It doesn't look too bad compared to some amps I've seen.

[5Trout]

New "original" schematic redrawn after the actual amp...

I'm trying to rebuild the preamp section at the moment
Maybe I will post a schematic of that later...
Is ExpressSCH files something that is used in this forum, I assume that they
are more readable than a exported jpeg for example?
In other words, should I post a .sch file or an exported image?