lowrey organ amp conversion

[Ran Randle]

Hey all,

I'm contemplating what direction to take with this amp. It has a 5U4, pair of nice vintage EL34s and a 6SN7 octal preamp tube with a 4ohm OPT. I wired an on/off volume pot to test it and it sounded good just not very loud. I could use some advice on getting more gain out of the 6SN7 and possibly adding another one. There is some hum but not too bad considering how old the can caps are. I plan to junk them anyway. I'd like to reuse the chassis but space is tight so adding another preamp stage will be a bit of a challenge. Ultimately, I’d like to get the most out of the existing components and tube complement without getting to crazy if you know what I mean. Keeping it simple is the way to go I think.

If this were your amp what comes to mind as far as a guitar amp conversion? Would adding another preamp tube for an additional gain stage be a good start? Would it be recommended to pull the trannys etc. and start over with a larger chassis? ...just putting the feelers out before I make any decisions, thanks.

I need to post some close up gut shots, sorry. I'll do that later tonight.

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[Ran Randle]

After doing some searching, putting in a coupling capacitor of .01 @ 400volts from the input to pin 4 of the 6SN7 should get the volume up.

[sgelectric]

Try subbing a 6SL7 for the 6SN7 for more gain..Pinout is compatible and mu for the 6SL7 is 70 as compared to 20 for the 6SN7.Kind of like the difference between a 12AT7 (60) and a 12AU7 (20).You will probably still want to add another preamp tube for additional gain,

[olddawg]

Did the power supply in this amp also power another sub-chassis? If it did you probably have plenty of extra heater current available. The PT looks pretty large. In that case I would consider using 12ax7s and
rebuild it using a turret/eyelet board and a known good guitar amp design. If space is a problem, build a single channel version of the amp you choose with no reverb or tremelo. You can also save some space and probably increase the B+ by using a solid state rectifier.

[sgelectric]

It would probably be a good idea to move the AC cord away from the input and reinstall it down by the power transformer somewhere..This is especially true if you intend to install another preamp tube in front of the phase splitter (6SN7).
(Edit) This post assumes that cable between the EL34s is the output and the input is the cable on the side adjacent to the AC cord.

[Ran Randle]

Thank you both for the helpful replies

I will try a 6SL7 as a phase inverter (replacing the 6SN7)and would still like to add another 6SL7 or 12ax7 for more gain. I will move the power socket away from the input.

There is a plug to power another outboard unit, probably a preamp of some sort for the original organ amp set up. The PT is beefy so should supply an extra set of heater filaments with no trouble.

Is it ok to use both 9 pin and 8 pin preamp configurations together, such as a 12Ax7 for first gain stage and the 6SL7 for the phase inverter? I don't see why not but don't see it very often, usually either one or the other but I know I've seen amps with both. That may be headed into complicating things a bit though.

This is the current circuit as of now:

[olddawg]

I will try a 6SL7 as a phase inverter (replacing the 6SN7)and would still like to add another 6SL7 or 12ax7 for more gain. I will move the power socket away from the input.

There is a plug to power another outboard unit, probably a preamp of some sort for the original organ amp set up. The PT is beefy so should supply an extra set of heater filaments with no trouble.

Is it ok to use both 9 pin and 8 pin preamp configurations together, such as a 12Ax7 for first gain stage and the 6SL7 for the phase inverter? I don't see why not but don't see it very often, usually either one or the other but I know I've seen amps with both. That may be headed into complicating things a bit though.

This is the current circuit as of now:

[ATTACH=CONFIG]15513[/ATTACH][ATTACH=CONFIG]15515[/ATTACH][ATTACH=CONFIG]15514[/ATTACH]

To tell you the truth, I would start over. You have the expensive parts. I would take the iron and what ever else you deem salvageable. I would recommend getting a new chassis, ceramic tube sockets, caps etc. Use a known good design, follow the layout. In the long run you will save yourself a lot of grief, time and headaches trying to cobble something together for lackluster results. It is very inexpensive to put together a turret/eyelet board. A fresh chassis can be had for as little a $25 or so if you look around. Ceramic sockets are cheap and you need to replace all of the electrolytics anyway. A standard size chassis also means a standard size cabinet. You have the makings of a great project for little investment. Start fresh and make something you will honestly be satisfied with IMHO.

[Gtr_tech]

That amp as it sits needs a line level signal (and I'd bet a hot one) to get to full output. Its just a PI and output tubes. So either use some sort of guitar preamp in front of it, or add the needed preamp circuitry to the chassis. Is there a sticky for this? If not, there needs to be.

[Ran Randle]

To tell you the truth, I would start over. You have the expensive parts. I would take the iron and what ever else you deem salvageable. I would recommend getting a new chassis, ceramic tube sockets, caps etc. Use a known good design, follow the layout. In the long run you will save yourself a lot of grief, time and headaches trying to cobble something together for lackluster results. It is very inexpensive to put together a turret/eyelet board. A fresh chassis can be had for as little a $25 or so if you look around. Ceramic sockets are cheap and you need to replace all of the electrolytics anyway. A standard size chassis also means a standard size cabinet. You have the makings of a great project for little investment. Start fresh and make something you will honestly be satisfied with IMHO.

I tend to agree, thanks for the honest input. I have a chassis with holes already drilled, large turret board, new sockets and 3 50+50 500v caps that would be perfect for it. I was planning to use it for a plexi 50 build and was considering buying new transformers spec. for that amp specifically to save myself some headaches but they are pricey unfortunately.

[Ran Randle]

That amp as it sits needs a line level signal (and I'd bet a hot one) to get to full output. Its just a PI and output tubes. So either use some sort of guitar preamp in front of it, or add the needed preamp circuitry to the chassis. Is there a sticky for this? If not, there needs to be.

Gtr_tech, thanks for the response. This may be a dumb question but what is a "sticky", like a reference tag or something?

I did try the amp with a mesa v-twin preamp(12ax7s) unit in front of it and it sounded pretty damn good but still not as loud as it should be (a result of the low gain 6SN7 phase inverter most likely?) So just for kicks, I might try the higher gain 6SL7 with the mesa preamp unit just to hear the difference.

[olddawg]

I tend to agree, thanks for the honest input. I have a chassis with holes already drilled, large turret board, new sockets and 3 50+50 500v caps that would be perfect for it. I was planning to use it for a plexi 50 build and was considering buying new transformers spec. for that amp specifically to save myself some headaches but they are pricey unfortunately.

Yeah... Modding point to point wired amplifiers can be maddening with lots of unintended consequences. (hum, layout problems, etc) Especially when it is a stand alone power amp like this and needs a whole preamp added. What you might want to do is measure all of your secondary taps and your raw B+ after filtration. That will give you an idea of whatkind of an amp you can build. If you need a little more B+ a solid state rectifier will get a bit more. Get a note pad and write it all down. But, I would guess that you can make a great amp with the parts you and maybe $50 - 75 in 12ax7s, jacks, components, switches and pots.

[Gtr_tech]

A "Sticky" is akin to a FAQ. Its a topic that always stays at the top of the list for frequent reference.

The Mesa preamp is intended for use in front of a guitar amp, so it has a low level output. A preamp that is intended to be used with a rack mount power amp will get you a hotter output, but still not hot enough to work properly with that setup. Rack pwr amps normally have a gain stage in front of the PI to bump up the signal level, so a normal line level signal will drive the amp to full output. That chassis doesn't have that gain stage....just a PI.

Subbing a 6SL in place of the 6SN will make little difference. Its not a gain stage, its a PI.

[Ran Randle]

A "Sticky" is akin to a FAQ. Its a topic that always stays at the top of the list for frequent reference.

The Mesa preamp is intended for use in front of a guitar amp, so it has a low level output. A preamp that is intended to be used with a rack mount power amp will get you a hotter output, but still not hot enough to work properly with that setup. Rack pwr amps normally have a gain stage in front of the PI to bump up the signal level, so a normal line level signal will drive the amp to full output. That chassis doesn't have that gain stage....just a PI.

Subbing a 6SL in place of the 6SN will make little difference. Its not a gain stage, its a PI.

Right, can't squeeze water from a stone. Thanks for that clarification, very common topic on this board indeed. So it goes.

[virgilvdog]

i virgilvdog like to know why there is a blank plug in the lowrey tube amp what is the purpose. because i have a lowrey amp that i can not get much volume from. i cut the wire from that socket that you have that plug in from the filter cap.

[virgilvdog]

i virgilvdog forgot to mention that my lowrey amp has 6l6 tubes i was wondering if the 6l6 tubes should be 6l6gc tubes because i was told that 6l6gc tubes was not made in 1959 and these two 6l6 amps i have were made in 1959 . info from the seller. i purchase from.thank you for any input.