Switchable 5F6a/JMP50

[jrwalsh33]

Hello,

For the music my son plays (think Roy Buchanan, Neil Young and Jimmy Page), he ideally needs both a '59Bassman (5F6a) and '71 JMP50 (1987). So, I am building him one amp head and a custom cab that will faithfully reproduce both.

I would be very interested in any comments about this, especially regarding the following challenges:

My design includes one instrument input and four main voicing switches: 1) Selects either the 5F6a’s 12AY7 preamp tube or the 1987’s ECC83 preamp tube, 2) Selects either the normal or bright caps on the selected preamp tube, 3) Selects either pair of the 5F6a's 5881 power tubes or the 1987's EL34 power tubes, 4) Selects 5F6a's GZ34 rectifier tube or the 1987's SS rectifier.

Questions: 1) Does the switchable preamp tubes and voicing caps, etc. require multiple inputs, or a push/pull volume knob?, 2) How to provide ideal primaries for both 5881s (4k) and EL34s (6.6k), and proper secondaries from these separate pairs of op tubes? Two relays - if so, what kind? Two OTs - if so, which and how to implement?

Any other ideas are most appreciated.

Joe

[Tubenit]

schematic?

It would be helpful if you had a schematic to post. I can't tell from the description if you're trying to build this to "switch" things while you're playing .... OR set it up so that you can change out tubes.

You can switch between tones somewhat by using a dpdt center off on the V1 cathode & paralleling caps and resistors. For example you can have 100k (plate) & 2.7k (cathode) with a .68uf in center off. Parallel a 10uf on one side and a 2.7k & 22uf on the other side and you have 3 different tones.

I think there may be a easier way of getting your son the different tones he is wanting? If you're just wanting a clean tone or overdriven tone .... there may be simpler ways of doing that. I personally would not consider using two OT's and switching between EL34's & 5881's. I'd just wire up the socket so I can change the tubes out and re-bias if you need both those sounds.

The other glitch you're gonna have trying to do this is notice the difference in values of the coupling caps and tone stack values between the Bassman and '87. Those account for a huge difference in tone. You could just build an '87 and then use the fat/full/bright switch I suggested & 5881's. Then if you wanted even a cleaner sound change the V1 to a 12AY7 and the phase invertor to a 12AT7.

This amp for example had enough switching that I could get clean tones to overdriven tones to it. (I have a layout for it also) I added a spst to switch the NFB on or off. In the off position, it was far more overdriven.

http://s28.photobucket.com/albums/c2...lexi25RMTS.gif

FWIW, my understanding is that Roy Buchanan often played a Champ mic'd thru a PA system. Neil Young reportedly plays a 5E3 Deluxe with 6L6's. Jimmy Page has played all kinds of amps from Supro to Marshall.

With respect, Tubenit

[Tubenit]

Editable schematic

If you would like an editable schematic to draw up your ideas, you can download ExpressSCH (free) and use one of these schematics to design your amp.

http://www.expresspcb.com/ExpressPCB...c_software.htm

Plexi 50w

http://www.el34world.com/Forum/index.php?topic=581.0

Bassman

http://www.el34world.com/Forum/index.php?topic=571.0

Plexi 50w w/reverb (has switchable bright/full/fat tone)

http://www.el34world.com/Forum/index.php?topic=580.0

With respect, Tubenit

[jrwalsh33]

Tubenit,

I very much appreciate your thoughtful response.

Yes, I want to switch between authentic 5F6a and authentic 1987 voices and functionality - not swapping - instentaneous switching. This, in effect, is like what all these "super versitile" "modeling" type designs promise but never really accomplish (too much tonal compromise).

My idea is to not compromise. Think of both amps in a single head cab. Then, combine all aspects of the circuits that are relatively identical (second and third preamp tubes, some resistors, some capacitors, the tone stack, etc.). As far as I can tell, that would leave the preamp tubes, voicing caps/resistors, power tubes, and rectification. Of course, B+, biasing, and secondaries would have to switch correspondingly.

My inclination at this point is to get Mercury Magnetic's best vintage-style 5F6A and 1987 OTs in there, and switch between the B+ voltage going to the primary of each OT. Although I would need a really unusually robust switch, I wouldn't need to switch between the outputs of the tubes and the OT, because each pair would go to its dedicated OT. In fact, with a sturdy enough switch (voltage-wise) couldn't I do it with one good DPDT switch - one side switches the B+ between OTs, the other switches/lifts the biasing connection. Am I forgetting something here?

Peace

Joe

[mr.jetski]

just my $.o2 but lifting the load off the ot under power would fry the works I believe.

[Tubenit]

Bassman/Plexi

I will be surprised if you can make your idea work in one chassis. You have these challenges:

1) not ending up with oscillation problems (MAJOR
concern & layout is critical)
2) switching all the coupling caps and entire tone stacks
while playing (this would require maybe a half dozen
switches or more changed at the same time)
3) turning off and/or switching out an entire power section
while playing. And if an OT is not connected to a speaker
then it will fry
4) changing between four 10" speakers for the Bassman
tone (which is intregal part of the sound) and the
four 12" Celestian speakers for the "authentic sound
5) switching a half dozen plus switches at the same time without any popping noises or other glitches

AND 6) for an uncompromising authentic bassman sound you will need to use Orange Drop caps and for the
authentic Marshall sound you will need Mallory caps. Either amp built with the other type caps will sound
different. I have built an amp with O.D.'s and changed everything to Mallory's of the same value. There is
a signficant tonal difference.

I would suggest you NOT begin buying any parts for the build until you post a schematic and layout for what you're going to do for others to review. I'd hate for you to build the amp and have major oscillation problems that made it a failure.

Take a look at just the Bassman tone stack and compare it to the Marshall. Do you see differences in the resistor, cap and pot values? Now go thru both schematics and circle every difference you find between those two amps. Now design switching and/or relay mechanisms for those to maintain the "authentic" sounds & "not compromise".

I love innovative ideas and creative amp designs, but I suspect you may find this amp idea an unlikely success.
It would be easier to build both amps and use an A/B/Y box.

However, using some of the suggestions I mentioned earlier might get your son somewhat similar to the Bassman and Plexi sounds, IMO.

You seem set on the project, so good luck with the build. I hope you will be happy with the results of your build and that you can get it to work out for you. I've done 16 builds, many of them my own design & your project would be way beyond my ability to make it a success.

With respect, Tubenit

[Tubenit]

A compromise

If you go ahead with this build PLEASE draw your schematic and layout up ahead of time, post it and get feedback on it. Oscillation problems are a nightmere to resolve at times.

Another option that is reasonable is to build two heads, a bassman and '87. Get a four 12" speaker cab. Hook the Bassman head to a pair of 10" Jensens and the '87 to a pair of Celestions. Use an A/B/Y box.

With respect, Tubenit

[pdf64]

As I see it, seemlessly switching the speaker from one OT to the other is the killer here - an inductive spike will be generated which will cause problems. Everything else is technically possible, although the altered lead dress due to wiring to the switching mechanisms will inevitably have a tonal consequence, even if the oscillation issues can be managed. Peter.

[jrwalsh33]

Tubenit, jetski, pdf, you guys are great.

Thanks for the admonitions. I am modifying my design as we discuss.

To be clear, when I say "instantaneous", I meant to stress "not swapping" but "switching". My design would surely require one to at least hit the standby switch first before other switching. I am thinking that would prevent the inductive spike, no?

I know of two OT stereo amps (e.g., Mesa) that avoid occilation, so I know it can be done...yes, very carefully.

Also, I believe the tone stacks are virtually functionally identical. It is the voicing of the caps/res/etc. that make the important tonal differences, which if I switch in accordance with the V1, PTs, and rectification would result in truly authentic sound (btw, I would also be able to combine say, the 12AY7 with the ELs, and SS, or other non-authentic combinations, for interesting voicing). BTW, I don't think the Bassman had Orange Drops. My specs call for NOS Astron .02uf or 100pf for NOS RCA 12AY7/ 320uf or .68uf for NOS Mullard ECC83, as in 5F6a/1987 circuits respectively, Mullard Foil-and-Polyester .022uf "Mustard" Coupling Capacitors for ECC83, Sprague Atom Electrolytic Capacitors, Selenium Rectifier, and NOS Tung-Sol 5881/EL34s.

The one concern that would definatly make me scrap this is if the switching will cause any tone sucking, as suggested by pdf64. I want to look into that more for sure.

All feedback still extremely useful!

Peace.

Joe

[Tubenit]

I wish you success with the build & hope you enjoy the process and results!

I have built an amp for my son also, so I can sure appreciate a father's heart to honor his son's ability and/or passion.

I hope you will take the time to draw up the schematic and layout for others to review. You will find it a VERY worthwhile endeavor. It may save you myriads of headaches. I even referenced you some editable schematics to use.

You may want to note some of the differences in the B+ rail between these two amps also such as resistor and cap values.

Best regards, Tubenit

[jrwalsh33]

Tubenit,

I really appreciate your positive and helpful vibe.

Thanks.

Joe

[MWJB]

jrwalsh - I think that you're making life unnecessarily hard for yourself. If you took a bunch of 5F6As and a bunch of JMP50s you'd find that they would all sound subtly different to each other (voltages would vary too), so to a point, I'd say that trying to faithfully recreate the schematics would be pretty pointless.

Most folks take the 12AY7 out of the 5F6a anyway and use 12AX7/Ecc83 (unless you're using a hollow body guitar with vicious pick ups). Though, you could build with 2 preamp tubes and maybe only use half of each (or use all of both tubes - layout problems may more be a problem at the other end of the circuit)?

Chances are that your son would prefer one channel from the JMP & one channel from the 5F6A. So this is what I would try...

half of V1 gets the 5F6A normal channel, with a 12AX7. The other half of V1 gets the input stage from JMPs preferred channel (switch with an A/B box).

Tone stack - switchable value for the treble slope resistor 100K/33K.

Switchable rectifer is easy enough. Use a PT with 345-0-345VAC secondary B+ winding.

Power tubes - I'd play safe and go with 1k 5W screen grid resistors & 1.5K to 5.6K grid stoppers, wire pin 1 & 8 together. Now you can run 6L6 or EL34, audition as many pairs as you can get your hands on until you find a tube that has the best compromise tone between the 2...the rectifier & higher voltage (& current) will change the tone anyway. Bear in mind current should be set with SS rectifier employed & you will just have to live with what you get with the tube, may only be +/- 5 or 6mA between the 2 at the end of the day?

Power supply & filtering - you could give each half of V1 it's own power supply node, voltage and filter cap value? Going up on the Fender values at main & screen supplies won't do any harm at all, 50uf at both will be fine (wire totem pole style like a super reverb). Use typical Fender & Marshal values at each half of V1. 20uf at the PI won't make a massive difference. Easy to have a switch to go from 4.7K to 10K at screen to PI dropping resistor...if you could be bothered.

This would minimise layout problems, still give you a tangible difference and still let you remain pretty faithful to both designs.

Mustard caps will work with both designs, plenty of decent sounding 5F6As have orange drops, where will you find NOS tubular foil Astrons?

Lastly, find a way to switch from 2ohm parallel output to 8ohms parallel-series/series-parallel (or 4ohms to 16ohms if using 16ohm speakers). Compromise on NFB resistor value, 56K to 75K?

[jrwalsh33]

MWJB,

Really helpful...I'm moving in the direction of two channels, and considering other advice from you and others. I agree that all amps of the same model sound different, but some of the sounds we desire are possible only from one of the two models, other sounds only possible from the other. I believe I will be definitely sticking with 12AY and 12AX both, think they are essential to desired range.

Thanks.

Joe

[MWJB]

"I believe I will be definitely sticking with 12AY and 12AX both, think they are essential to desired range." Well, stick with the preamp Fender circuit & voltages (170-180v on the 12AY7 plate), but I'd remain flexible on the specific tube that you end up with...most 5F6As do NOT have a 12AY7 in V1, so I can't agree that 12AY is essential to "real world" 5F6A tone. Considering the tube can be changed in moments, it seems a little back to front to be dead set on a specific tube, rather than the conditions it lives in. The preamp tube that sounds best is the one to use.

[vintagekiki]

For 4) Selects rectifier ( tube or the SS ) - see schematics for Mesa Boogie Dual Rectifier power supply.