Seymour Duncan Convertible 100 Head

[plexi50]

Im not much into the modules they are using these days in tube amplifiers

But i am curious about this amplifier. I have looked at the power and preamp schematics including all modules and there values

Looks like a bit on the nasty side for some of the plate resistors used and odd cathode resistors in the moduels

A different bird  / I have the opportunity to trade one of my 18 watt builds for it

Does anyone have any goods and bads to add about this amp? Built in 1987

This pic is the combo model.

[Joe6v6]

A few years back a friend gave me one to use for parts,I still have the transformers, during the dismanteling it was very clear that this amp was just a problem waiting to happen. Some of it seemed well built but the socketts for changing the modules were poorly designed and frought with problems. The guy told me that over the time he had it it was in the repair shop more than it was used for gigging. He did say that he like the sound and tone of it during the rare times it was operational..  ..     Joe

[plexi50]

Thanks for the info JOE. If nothing else it will give me another project to possably fabricate better moduel sockets

I have read good things about the tone online so i supose i will do the trade tommorrow Sunday

It is in good working condition so i dont have anything to loose to start with until i get my hands in it that is

I am nortorius for new ideas,fabricating and overkill / Steve can vouch for that!

[plexi50]

What type of PI is in this amplifiers? Schematic here: http://www.triodeamplification.com/files/SDC-Powers.pdf

[danhei]

This site has a lot of info on the Convertibles: http://www.diyguitarist.com/GuitarAmps/Convert.htm

[PRR]

> What type of PI is in this amplifiers?

Simple clean long-tail.

V2 common cathodes and constant-current "tail" D5 take input at either grid and deliver to both plates.

Balance should be very good. Gain is abit weak for the overall scheme and output would be a little weak for four fix-bias EL34s, so V1 is a driver. Not a lot of gain, potentially high output, and as a bonus, it has some re-balancing action in the common cathode resistor.

What puzzles me is V3. It has no signal at grid or cathode. Its plates load V1. As I read the jack, with no plug the grids can be swung from zero to -22V. At -22V they are "off". At zero volts V3 is "on", steals current away from V1, also loads V1's plate output. It may be an odd form of master-volume.

The switching for the screen grids, and the PT HV connections, must be miss-drawn.

[HotBluePlates]

A different bird  / I have the opportunity to trade one of my 18 watt builds for it

Does anyone have any goods and bads to add about this amp?

From a collector standpoint, you can't give that amp away. So you're probably on the wrong end of a very one-sided deal trading an 18w for it.

I know that I saw a pair of those amps languishing in a couple of Nashville vintage shops for years with no takers, while the rest of the invetory changed quickly. I also heard that they supposedly sounded good, but I also never saw anyone play through them. That told me all I needed to know.

[plexi50]

I just read your post HBP and i concure. I was posting this and stopped to read your post before posting this earlier:

Man this thing sounds like crap no matter what moduel is switched around. First of all i see that in the schematic that this is plate driven amp. For me that wont work using EL34's. Too harsh. Cathode driven is smoother and can be more defined and tweakable. Im just a cathode lover thats all

Second / Looking at the moduel schematic's (Parts layout inside) for each moduel is pretty cheesy on the values used and there is definatley no love in the tone. It is loud and will sustain for days but i dont care about that. Im after that smooth growl and clean channel tone. This has the most peanut brittle tone i have ever heard

It would be easier to just yank the board and put a turret board in it but i see some hope here in possably doing a whole lot of moduel rebuilding and some board changes

Then again i think (Just sell it) and move on. But the challenge is here

Havig a set of dual tone stacks and switching already is a big plus (+) to me so i am thinking once again JTM-45 and Marshall 2204

The transformers are HUGE and the B+ is 575 VDC off the standby / It could be a great amplifier but will require a whole lot of changes

[HotBluePlates]

Your Tek 515A was one of the cool compact models, and so it did not have a plug-in vertical amplifier (the section to the far left, under the CRT, which determines volts/division).

I once dreamed about making an amp with plug-in modules for the preamp, using the same connectors that Tektronix used in their scopes. The scope body had a female socket with maybe 12-24 contacts, half on each side (I don't recall the exact number). The plug-in had a corresponding male plug, and the plug-in chassis essentially had a very long screw with a knurled knob on the outside, a straight shaft, and maybe 1/2 inch of thread on the inside. You slide the plug-in in place, and (although you didn't necessarily need to) tightened down the knob to lock the plug-in in place.

It's a cool idea, and supply voltages can be contained within the chassis for safety. But how you execute the idea is critical, as I think you see. There has to be a lot of mechanical and electrical excellence to pull the thing off.

[plexi50]

Now it's funny you mention the socket idea. I have just parted out 3 organs and i have i think enough female and males to work with. As we know they are made of good old metal and fit very tightly together. Though i am not sure which plan i am going to come up with and commit myself to yet. Any way this goes is not going to be fun. It's the outcome though that could be really slick

[plexi50]

I have a bunch of these connectors as well /

[Dave]

Be careful admitting to gutting old organs. I got bum-rushed once for admitting the same thing.

Dave

[plexi50]

Ha! I was thinking of that sin. But these were all solidstate so there wanst much inside anyway. Anyone want to be an ORGAN donor?

[HotBluePlates]

I just looked at the website with all the convertible information, and the sockets for the modules is similar to the Tek female sockets. Same kind of form factor, but the Tek parts were larger and made a bit differently to be more structurally sound. Second, they were mounted vertically rather than horizontally, and the integrity of the connection was not depedant on the socket holding the plug-in in place. That's what the front panel locking shaft was all about.

For me, the way I'd do it would be either an entire power amp and phase inverter as a set configuration, with a complete preamp module that slides into the chassis, or at most a preamp module and seperate tone control module. But I think I'd lean heavily to a complete preamp, so everything would be known to work together well.

I might actually talk myself into making this, but the metalwork is what would hold me back. I think there are a number of places to find to plugs and sockets, or use your octals (assuming that provides enough contacts).

If I were building it, I would go with a head format, and build the phase inverter, power amp and power supply (up to the point of feeding the phase inverter). The cavity to slide the plug-in would be nearly as deep as the chassis (which need not be too deep, but might be somewhat tall), which the connector very close to the phase inverter tube. You need B+, filament (x2), ground (perhaps more than 1; power and signal ground) and signal pins, and possibly more than 1 signal pin. So 8 works but 10-12 would be better for anticipating unique arrangements. You want this connector near the phase inverter tube to keep wire runs as short as possible.

The plug-in would need its own decoupling/filter caps. The plug-in has to have its own subchassis to ride in; look at Tek plug-ins for an example. And you need some form of locking mechanism to hold the plug-in in place without relying on the socket to do the job.

If the main chassis circuit is conceived with the right required input level for the phase inverter and the plug-in circuits designed to supply that level without going too much over, it should work beautifully.

Did I mention I'm talking myself more into this, aside from the fear of figuring the mechanics of it?

[plexi50]

Thats what i am doing. Trying to talk myself into do it.  It would be so simle just to drop a turret board in it and wire it up. I could join the two channels with a 470K resistor and A/B box to switch between the channels. It sounds like i am trying to talk myself out of doing this but im not.

It's just scrammbling my brains thinking of the possabilities and the many ways to do this and do it nicely

Sometimes thinking too much about something holds you back from digging in and getting dirty

This aint no spacecraft but it looks like one / Anyone feel like doing a cathode layout?

http://www.triodeamplification.com/files/SDC-Preamps.pdf

 

[HotBluePlates]

I still wouldn't trade your completed amp for it. Instead, I'd build something that demonstrated the concept, but executed better than the Seymour Duncan.

[plexi50]

The 18 watt i traded was not a complete head with cab. Just a small 1930's chassis with a pair of 6V6's. I still have another. If nothing else i have some good transformers and another chassis with cab to build from. Man i still can not believe how bad this thing sounds. The moduels are a joke. The part values in each of all but one of them are no where near what you could call high gain. Maybe there is something else going on in the amp. Im not gona sweat it. Maybe there is a HiWatt and Marshall in it's future

[Bickster]

Plexi50,
I had one for years.  It was quite un-reliable.  When I finally gave up and parted it out I found all kind of smoked resistors that were buried in hard to access places.  I gutted mine and sold it for parts.  Very nice iron.  The modules are worth more than the amp!

Bob

[plexi50]

Well i did a lot of reading last night on the problems this amp was known for and the fix's for it

I have done the Marshall HiWatt Orange EQ mod and the EQ is now after the preamp

I also changed the first preamp plate resistor from 47K to 100K / Changed the master volume 47K's to 100K to roll off the highs better

I changed out the high gain channels slope resistor to 33K and left the clean channel 100K

I removed the 2.2M resitors from the 250pf tonestack treble caps

I will be turning it on and checking out the tone later this evening if not sooner

One of the main problems i found was there were 3 12AX7 tubes in the wattage selector circuit

They are suppose to be 12AU7-12AX7-12AU7 / 3 12AX7 tubes is the cause of too much nastyness (Nastyness?)

I put in (2) 1954 NOS RCA 12AU7's and one 12AX7 as it is suppose to be

Now if it sounds worth keeping i will contine and modify each of the moduels

Channel one will be a plexi circuit and Channel 2 will be Marshall high gain

Can i remove R11 R12 R14 1Meg? http://www.triodeamplification.com/files/SDC-Preamps.pdf

QUESTION: This is not critical but should i go ahead and change the grid resistors from 2.2K to 5.6K?

Are the 220K plate resistors in the PI the reason for using the recommended 12AU7 tubes in the so called SD wattage stage?

Would i benifit by just changing the 220K PI plate resistors out to 100K and using 12AX7 tubes?

  

 http://www.triodeamplification.com/files/SDC-Powers.pdf

[HotBluePlates]

Can i remove R11 R12 R14 1Meg?

Presumably, those are grid reference resistors. So, no.

But the big drawback in the schematic is that a generic amplifier symbol is used, without regard to what is inside of the modules. I would also assume the numbers shown are conyact numbers of the module, not pin numbers for the tube, so you can't be 100% sure of what's going where without more info on each specific module.

This is not critical but should i go ahead and change the grid resistors from 2.2K to 5.6K?

I'd leave them unless you had instability that you were certain was caused by too small grid stoppers.

One of the main problems i found was there were 3 12AX7 tubes in the wattage selector circuit

They are suppose to be 12AU7-12AX7-12AU7 / 3 12AX7 tubes is the cause of too much nastyness (Nastyness?)

According to your schematic, that's supposed to be 12AX7->12AU7, with a 12AX7 wattage control stage.

Are the 220K plate resistors in the PI the reason for using the recommended 12AU7 tubes in the so called SD wattage stage?

Would i benifit by just changing the 220K PI plate resistors out to 100K and using 12AX7 tubes?
I wouldn't touch anything here unless you can be certain you will improve things. It's not a Fender-style stage, so don't try to force Fender values just because they're familiar.

Actually, a plate load around 47-56k would be more typical with a 12AU7, but there's obviously some regard given to the 12AX7 attached. Figuring what's optimum for an unusual circuit like this would take a good deal of time and a lot of scratch paper, so again, I wouldn't fiddle here. Just fix things that are obviously broken, or sh*t-can the whole wattage control and variable damping (which is really variable feedback, but whose values are tied to values used in the phase inverter circuit).

[plexi50]

Can i remove R11 R12 R14 1Meg?

Presumably, those are grid reference resistors. So, no.

But the big drawback in the schematic is that a generic amplifier symbol is used, without regard to what is inside of the modules. I would also assume the numbers shown are conyact numbers of the module, not pin numbers for the tube, so you can't be 100% sure of what's going where without more info on each specific module.

This is not critical but should i go ahead and change the grid resistors from 2.2K to 5.6K?

I'd leave them unless you had instability that you were certain was caused by too small grid stoppers.

One of the main problems i found was there were 3 12AX7 tubes in the wattage selector circuit

They are suppose to be 12AU7-12AX7-12AU7 / 3 12AX7 tubes is the cause of too much nastyness (Nastyness?)

According to your schematic, that's supposed to be 12AX7->12AU7, with a 12AX7 wattage control stage.

Are the 220K plate resistors in the PI the reason for using the recommended 12AU7 tubes in the so called SD wattage stage?

Would i benifit by just changing the 220K PI plate resistors out to 100K and using 12AX7 tubes?
I wouldn't touch anything here unless you can be certain you will improve things. It's not a Fender-style stage, so don't try to force Fender values just because they're familiar.

Actually, a plate load around 47-56k would be more typical with a 12AU7, but there's obviously some regard given to the 12AX7 attached. Figuring what's optimum for an unusual circuit like this would take a good deal of time and a lot of scratch paper, so again, I wouldn't fiddle here. Just fix things that are obviously broken, or sh*t-can the whole wattage control and variable damping (which is really variable feedback, but whose values are tied to values used in the phase inverter circuit).

According to your schematic, that's supposed to be 12AX7->12AU7, with a 12AX7 wattage control stage

Getting tired and did not pay attention to my typing. You are right

Well it all comes down to the moduels and like you said / Yes the pins on the socket are not in exact reference to the tube socket on the moduels. My perspective of the tone is all the moduels need to be rebuilt to standard Marshall stages and then the amp should sound much better. It sounds decent now but i have too many gainy moduels and only (2) Classic Tube moduels. All the other moduels need to be made as they would be in a normal tube amps grid & cathode values



 

[plexi50]

I pulled the 12AX7 tube that operates the Variable power and the amp sounds much better. Too much unwanted feedback as you stated HBP. Not really a variable wattage control. The reverb does not work so i suppose i am looking at replacing the reverb chip. I can gear spring action but nothing in the tone signal. Removed the tank checked it out on an ohm scale and it is good. Cant find any info on this OC Electronics reverb tank. The cables are good

[HotBluePlates]

Not really a variable wattage control.

That's "Marketing!!!" A lot of things in the guitar amp world aren't exactly what they claim to be, at least in modern amps.

I look at it like this: Say you have a power amp that with any signal your preamp/phase inverter can deliver, you just cannot drive it hard enough to distort. Also, say that your preamp has all kinds of facility to create a distorted sound. If your amp got that distorted sound at less volume, and you didn't know anything about electronics, and the control was marked "variable wattage"... well you could easily believe the actual power of the output stage was turned down. It all sounds the same to you.

So really all they have is a master volume control. But you're on stage and don't want to dance back to the amp in the middle of your solo to turn up the master. You want a footpedal you can place on your Pedalboard of Doom. You and I know that you can't just run 2 wires from the phase inverter out to a volume pot in a wah pedal, and another 2 back to your output tube grids. Because it will make the best oscillator or AM radio you've ever played guitar through.

So that's where this clever idea comes in, which is not unlike how you might control volume using a solid state device. If you run a d.c. control voltage out the wires to your pedal and back to the amp, and they never directly feed into a signal-carrying stage, you don't have any noise problem. That extra 12AX7 is being used as a variable impedance to turn down the volume of the 12AU7 phase inverter. It will work, might not sound great, and could be done more easily and cheaply with a control voltage controlling JFETs attached to a different part of the circuit. Might even sound better.

Variable wattage? No, even though the label claims it. But it is a dandy way to have a remotely-located master volume. Even if it sounds like crap.

[plexi50]

It's been a wacky trial and error day. I rebuilt 4 of the moduels to lower gain values. Man what a hard time it is trying to remember and follow all the traces rather than just have a moduel whoms pins are in relation to the tube itself. Anyway the amp sounds dam good at this point and is smooth and punchy rather than Bee's flying around. The OD knos can really bring in a sustain for days if you like that. Again it is smoother now and more like a Marshall than it was before rebuilding the moduels

They used clipping diodes in the Classic Distortion moduel and the Hybrid Tube moduel is the last one to do. I wonder what is in that one. A loaf of bread? I have all the moduel schematics except for that one. Ill get to it tommorrow

I will say this has been a cool but tiresome learning experince with this amplifier.  No one has called the cops yet /

[plexi50]

Finished the final Hybrid Tube moduel today. The preamp channels are as follows:
Clean Channel has a total of (3) Classic moduels. Input moduel is 1.5K/.1uf cathode caps and (2) stages of 1.5K/25uf cathode caps

The Dirty channel has a total of (3) moduels. The input is moduel is 1.5K/.1uf cathode caps / one 1K/4.7K and one 1.5K/25uf

In that order. It sounds excellent to me on both channels operation. Now i can get away and go on a vaction. Not really. All the moduels except for the Classic moduel i found to have clipping diodes and stupid stuff in them
 
The preamp board itself has 100K plate resistors on it. Only the 1K/4.7K moduel has additional 100K plate resistors in it

The original had 470K plate resistors. Insanely rediculious. The fine tuning is over and the rest of the amp has been modded to my satisfaction as far as the EQ being at the end of the preamp now / The masters and OD controls work perfectly and are well balanced

I am glad i did not gut the amp and go with my usual gun ho manner /

[plexi50]

I have no reverb in the signal. I can hear the reverb spring action if i move the amp though. Is this one of the chips that operate the reverb? All cables and tank are good.

[JayB]

I can see why that thing sounds like garbage. Nothing wrong with the tone stack being fed by the plate. They just did it wrong. Done right, the only difference is the compression. Just about everything makes me wonder what they were thinking. That's like some of Rivera's early fender amps. Some of those amps I have still have no idea what they were thinking.

[HotBluePlates]

According to the schematic, the reverb is driven by an LM380, which looks like a 14-pin IC. Easy test would be to unplug the tank, attach your meter to the jack and see if you get any a.c. when you play through the amp. If not, then try checking the input to the IC, which looks like pin 6.

[plexi50]

I have 5.49 VDC on the LM380 Chip and 8.45VDC on the 5534 Chip. As i play: No voltage on the reverb Send / The return is active with plenty of spring action. Going over the connections and pots

My mistake above. I had to get the data sheet on the LM380N chip to correctly locate pin #6 on the LM380N chip

NO voltage on pin 6

[HotBluePlates]

Well, look at that mix circuit that comes from the channel 1 & 2 outputs and encircles yje whole reverb circuit.

First, do you have the footswitch and is the reverb engaged? You say you get spring noise out, so I would assume that's a yes, but I figure I'll ask the dumb question first.

Assuming you do, and asusming no signal on the input of the LM380, I'd be looking at R23-R28 to make sure they're good. It seems like you should at least have the presence of some kind of signal on the input of the reverb driver.

[plexi50]

I noticed some thing last week. This schematic is a little different in the it shows a reverb switch and master switch. Mine has niether. I noticed that the schematic is Revision A. I dont know if that is the first run of these or an update to newer ones that were changed. This amp is dated January 1984 from the pots and January 1985 from a sticker on the chassis

I made it easy to end any more fiddling in this amp. I removed the 100K reverb pots and put 2 250K pots in there place for Raw pots for both channels. More clean headroom

I also eliminated the effects loop and removed the reverb from the circuit.  Tone suckers anyway. If i want reverb i will use a pedal

Each of the shielded cables (4) total from effects and reverb to there plugin points were 24"s in length each

The best thing about removing the effects loop is i only had to put a jumper pin right next to another to reconnect the preamp out to the power section

Amp gets some real nice tonestack interacation with lifting the tonestack at variable degress

The last thing to do is to make that wattage knob on the front panel a PI damping pot for PI voltage control pin's 3 & 8 of the PI tube

Scratch the PI VDC pot control. I just put a 12AT7 in the supposed wattage circuit and is working amazingly nice

I quess it must have had a bad moduel in the begining before i rebuilt them that was the cause of the horrible tone at that time

It sounds great from the 5 watt position all the way up to the 100 watt rating and is very noticable and functional at this point

[Voxbox]

HBP,

I used to own a Tek 535 - it was beautifully built and I think the connector you have been referring to was made by McMurdo?

Hope this helps.

Cheers

[HotBluePlates]

It very well could be! Funny thing is, I've got a bunch of tube Tek scopes if I can muster the will to destroy them to steal parts...

[dusk]

  :w2:Hi i know this post is old,but i am taking a chance,i have the 100 watt head version and would like to make the eq mod, Plexi50 could you tell me if you used hot glue on jointing the 0.01uf cap to the circuit wire

thanks dusk

[plexi50]

Dusk i don't think so. I just changed out some parts and laid them in place.