RCF AM 10 (transistor+tube PA amp) How to convert it to all tube guitar amp ?

[kagliostro]

I got an old RCF AM 10 - 15W amp

the circuit uses transistor for the first part of the pre

one pentode for the final part of the preamp (V1a = 1/2 6AN8A)

one triode for the PI (V1b = 1/2 6AN8A)

two pentode for the power amp (V2 and V3 = 2 x EL84)

I hope that the heater winding can afford to supply one more tube

ave you any idea about how to arrange this further tube (I would like to take off the transistor circuits)

here the schematic without component values and attached a link for the complete schematic (who is near 2.4Mb) to download the pdf file you must click on <Download> on the upper side on the right

http://www.mediafire.com/view/?zdejo45xju5mwaj

Or do you think is better to save only PT - OT and Final Tubes ?

Thanks for any advice

K

[tubeswell]

The 6AN8 filament draws 450mA  http://scottbecker.net/tube/sheets/127/6/6AN8.pdf

The triode's amplification factor (21), transconductance (4,500 uMhos) and the plate dissipation (2.8W) are higher than a 12AU7, so it'd provide plenty of current as a cathodyne (which is what it functions as in this amp).

And the internal plate resistance (170k) isn't that high for a pre-amp pentode, so it'd probably be okay with a simple tone control and vol load.

Methinks you could make an amp just with that tube in the pre-amp/PI and the EL84s in PP.

Alternatively, you could use an EF86 pre-amp and probably get by adding a dual triode (for a TS recovery and Cathodyne, or probably better yet for this 2 x EL84PP amp;- an LTP), for a total pre-amp heater winding draw of 500mA (as opposed to 450mA for the 6AN8)

[kagliostro]

Many thanks Tubeswell for your info and proposal

I remembered that Sluckey, some time ago, indicated Sunn as a brand that was using the 6AN8 tube in that position, I've done a little research and I've found this

http://home.comcast.net/~seluckey/amps/sunn/sunn.htm

http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

the final tubes of my amp aren't 6550 but may be something can be borrowed from the Sunn architecture

K

EDIT: I've just find also that the Sunn Sonic I-40 uses EL34 tubes (that are similar to EL84 as tone) and the same architecture
http://www.prowessamplifiers.com/schematics/post/sunn_sonic_i40.pdf

[tubeswell]

There might also be a bit of 'wiggle-room' (in terms of heater winding draw) in the lamp (depending on whether you have a 500mA, a 300mA, or a 150mA 6V lamp in there)

[kagliostro]

There might also be a bit of 'wiggle-room' (in terms of heater winding draw) in the lamp (depending on whether you have a 500mA, a 300mA, or a 150mA 6V lamp in there)

Nice Idea !

I think one of the lamps is a neon lamp, I must control, but if there are filament lamps I can put led instead of it saving some current

K

[DummyLoad]

build a XLR male to 1/4" female phone jack and plug in your guitar. looks cool just as is.

butt... if you must... remove SS stuff from R46/R47 to the left. remove the PS rail for the SS stuff: leave C31 and remove everything to the left. install fender deluxe preamp & PS string & annoy neighbors.   

your PT should be to handle a couple of additional pre-amp bottles without issues.

you should use sub-miniature tubes for the pre-amp... 

--DL

[PRR]

> I would like to take off the transistor circuits

Why?

It is a fine microphone amp the way it is.

For guitar, disconnect the input transformers and remove R3. That is a nice high impedance clean path with volume, bass and treble.

The Reg/Fono jacks, replace the switch with two 10K resistors, you can put your iPad or CD player in here and do karaoke.

If you de-transistorize it: what is left looks a LOT like a Dyna or Sunn power amp (also late Ampeg VT-40). More gain than a Fender/Marshall triode driver. I would leave that alone; it's fine. The sensitivity at 6AN8-p grid is probably 0.2V-0.5V, so you will need a preamp, perhaps stolen from Sunn or Ampeg. Typically triode, volume, triode, tone-stack.

> XLR male to 1/4" female phone jack

I think the input is intended for 150-ohm microphones. Even with unloaded transformer it is probably far too low impedance for guitar. Also there is R3, which suggests TR3 is a 1:1 and the input is just about 470 OHMS, not the > 470 K_Ohms we expect for guitar work.

[kagliostro]

One clarification

the amp in question haven't input transformers

I've one other amp (RCF AM 20) who has the same circuit but power tubes are EL503 in UL configuration, this one has the input transformers

may be RCF has build some different versions of the AM 10

K

[DummyLoad]

Even with unloaded transformer it is probably far too low impedance for guitar

the amp in question haven't input transformers

thought so. would have likely carried a "TR" ref. des. as other inductors in schematic do. i think it's "J1" for "jack 1".

R3 is not in ckt. w/ SW1 in pos. 1 as shown.  maybe you meant R1? or do you mean put SW1 in pos "3" and clip out R3?

[TIMBO]

Hi K, I'll try to explain what i have drawn  In my BTO ECL84 i tried to build a MINI version of the BTO and in the original BTO they used a 6AN8 in the FUZZ circuit.But in my FUZZ circuit i used a 6AU6 for the PENTODE part of the FUZZ circuit, this had a simple tone and volume that followed it and thanks to one of DLs circuit i was able to get it up and running  :worthy1:I also had little mAs from my transformers so i was happy with the sound produced. Then i started thinking about the original BTO that used the 6AN8 TRIODE after the pentode as a follower. This i think was to help boost the siginal ( someone with more knowledge on this subject might add more)so i added the IRF820 as we have done in many builds with great success and for what it has added to the circuit i think it is worth it. Darryl used one of the triodes of a ECL84 as a PI in similar way so rather adding another tube leave the PI and POWER TUBES as is and build a PRE AMP around the PENTODE. Some of the component values may need to be tweaked. Hope this helps. 

[kagliostro]

Hi TIMBO

thanks for the schematic and for sharing your experience

can you post a link for the BTO schematic ?

at the moment this seems the more probable "donor" schematic for the mod (Sunn Sonic I-40)

K

[sluckey]

Why do you want to use the I-40 schematic? That's a BASS amplifier. And it's the only old Sunn that uses a 7199 rather than a 6AN8. If you want a guitar amp with no trem or reverb, take a look at the 100-S. It's actually voiced for a guitar and has a really good clean sound. And it uses the 6AN8.

Take a look at page 2 of this pdf...
http://www.el34world.com/charts/Schematics/files/sunn/sunn_100s.pdf

[kagliostro]

Why do you want to use the I-40 schematic? That's a BASS amplifier. And it's the only old Sunn that uses a 7199 rather than a 6AN8.

Because I'm very forgetful, I didn't recognize that the tube was a 7199 

I only noted the use of el34 that has tone similar to el84 and the pentode/triode tube 

V1 in 100-S is very close to the Sceptre that was the first amp I remembered to have a 6AN8

Thanks Sluckey

K

[sluckey]

The Sunn Solarus is an EL34 guitar amp. But, the only difference between Sunn's EL34 amps and their 6550 amps is the PT and OT. They both use identical schematics (except for small differences in power supply. It's kinda like Fender's AB763 line. Fender used the same schematic for all the AB763 amps, but used different size iron and speakers.

[PRR]

> R3 is not in ckt. w/ SW1 in pos. 1 as shown

I think the JPG differs from the PDF?

[PRR]

> 7199 rather than a 6AN8

Not a lot of difference (maybe pinout?).

The 100S topology (gain, volume, gain, Fender-ish tonestack, power amp) would be appropriate for this high-gain power amp.

[DummyLoad]

> R3 is not in ckt. w/ SW1 in pos. 1 as shown

I think the JPG differs from the PDF?

it does. no input trafo on the jpg. xsistors are diff as well. BC114 on the .jpg format schema. BC107 in pdf. schema - not that it matters much.

KL should try it as is. just lift the input (R1-R3) network. i'll bet it makes some killer distortion. 

--DL

[kagliostro]

Many thanks to all

DummtLoad do you mean I must connect the "wiper" of SW 1A directly to pin 3 of the input connector (an old DIN, not xlr) ?

What about SW 1B ?

K

[kagliostro]

I was thinking about this amp conversion and I ended with the idea to use the Sunn 100-S schematic

that will give good results for sure

Than I discovered that I've (coming from one other amp of the same brand) a recovered 1842 PCB

on that PCB there is mounted the 6an8 circuit and this PCB is the same as that mounted on this amp

an idea comes to me, why don't use this spare 1842 PCB as V1 & V2 for the mod on this amp ?

V1 = 6an8 pentode > V2 = 6an8 triode connected as CF > Tone Stack > V3 = 6an8 pentode as gain stage > V4 = 6an8 triode as concertina PI > V5 + V6 = el84 as final tubes

but I'm not sure of the convenience of this choice

What do you think about tone results ?

it will be better to copy V1 & V2 from the Sunn 100-S or to use one other 6an8 tube ?

till now I've seen 6an8 pentode used as V1 only in HiFi schematics

(like in the Dynakit)


so .......... what do you think about ?

Many Thanks

K

[sluckey]

That Dynaco is a power amp only. The 6AN8 is the PI. If you compare that Dynaco schematic to the Sunn 100S schematic you will see that they are exactly the same. Sunn was born from Dynaco.

[kagliostro]

That Dynaco is a power amp only.

  .....   ........... YES, of course you're right

so the only schematic I've with 6an8 as V1 is a modulator (see attached schematic)

K

[DummyLoad]

give this a shot before you throw out the SS stages... you may be pleasantly surprised.

--DL

[kagliostro]

OK DummyLoad

I'll repair the amp (there are some broken components) and I'll give a try to your schematic

K