A Million Questions - Traynor mod/rebuild

[gazinkus]

Latest issues:

1) I swapped out tubes, one by one, until I found the one that would start misbehaving after about 2 minutes of applied B+ . It was the phase splitter tube. Replaced it, and all is well.

2) I measured the power supply voltages. It appears that my setup is a bit hotter than the 66 and 69 Traynor setups, with +445 max, and +410 for the preamp anode supplies (!). I think I've determined that this will drop to about +367 (I need to up the wattage of a PS resistor...) with my anticipated load. Here's the issue, assuming that I indeed get a +367 out of this rig prior to the anode resistors on my 8 planned preamp stages (1/2 12ax7 each). I believe that the anodes want to see a specified voltage in order to behave per the design(s) I am emulating. Accordingly, if I have 367 volts at the supply side of the anode resistor, and I want to get to, say 200 volts at the anode itself, I need to drop that 167 volts across the resistor. Ohm's law tells me how big the resistor needs to be, if I want the anode current to be the specified value. So, even if the design I am emulating has a 100K anode resistor there, to get the anode voltage and current of the original I might need to put in a 200K resistor there, because I have a hotter supply than the emulated circuit. Right? Right?

Thanks to all you experts...

Dennis
Wimberley, TX

[Steve Conner]

Hey Dennis, glad you're having fun :-)

It's probably not a good idea to change the plate resistors like you said, because it also affects the AC gain of the stage.

If you look at your amp's schematic, you should find that the B+ rails to the various preamp tubes have RC filters on them for decoupling. The "R" is usually 10k or 27k or whatever. To lower the plate voltage of a preamp stage simply make this R bigger.

On a similar note, you ought to check the actual line voltage at your location relative to what the amp's voltage selector is set to. I hear that line voltages have got hotter over the years in the US, and maybe other countries too. (I live in the UK, which is nominally 230V now, but my line voltage actually measures between 240 and 250V, so I crank those voltage selector dials up as far as they'll go.)

steve

[gazinkus]

OK, but...

Thanks, Steve...

Yeah, it's a hoot so far. I'm finding out just how little I know about something I've been around all my life.

It's probably not a good idea to change the plate resistors like you said, because it also affects the AC gain of the stage.

OK. I can buy that. And, it's also why I asked the question. But, it begs another question - what the heck is the difference in changing the resistance between some B+ value and the anode of a tube at one resistor or the other - they're essentially in series. And, by adjusting the 8 individual anode resistors, I can specifically tailor the anode voltages to what the designs seem to expect - at the tube.

If you look at your amp's schematic, you should find that the B+ rails to the various preamp tubes have RC filters on them for decoupling. The "R" is usually 10k or 27k or whatever. To lower the plate voltage of a preamp stage simply make this R bigger.

Yeah, there's a 10K between the phase splitter offset voltage and the feed voltage for the anodes. In the Traynor, this latter voltage goes straight to the 470K anode resistors for all of the parallelled input stages and a 220K for the mixer stage. I'm replacing all of that with a Plexi-type channel and a Dumble-type channel. Best I can tell, the Plexi wants to see something like 191 or 161V at the plates, depending on the stage, and the Dumble wants 206 to 229, depending on the stage. I've calculated the anode current that each stage wants to see per the designs I'm ... uh ... borrowing. It totals about 7.3mA.

My first thought was to just double that 10K to 20K and drop some volts there, and I guess that's what you're suggesting. But given that the Traynor supply seems to be giving me 410 (I measured it with a crappy analog meter and the existing PA circuit with one 12ax7 gone), I'm wondering what it will really give me when I start loading things.

Has anyone just dropped the V's with zeners? Sure would take a lot of guesswork out of things. I could rip out that power resistor, put in a couple suitable zeners and caps, and end up with EXACTLY the voltage I want at the heads of the anode resistors...

On a similar note, you ought to check the actual line voltage at your location relative to what the amp's voltage selector is set to. I hear that line voltages have got hotter over the years in the US, and maybe other countries too. (I live in the UK, which is nominally 230V now, but my line voltage actually measures between 240 and 250V, so I crank those voltage selector dials up as far as they'll go.)

Well, I'm kinda lucky there. This is a single-supply model with no voltage selector. US only. So, whatever I do has to be accomplished in the secondary. Just for grins, I just checked the line voltage AT the power jack of the amp... 122 VAC. The '69 schematic specs 115VAC. Mine isn't the same as the '69, but isn't all that different, either.
Dennis

[Enzo]

Unless this is a finely tuned machine, I might suggest worrying less about the voltage. It is real typical for voltages to be a lot higher in old amps now than they were back when, yet they still sound OK.

But given that you want your target volts:

what the heck is the difference in changing the resistance between some B+ value and the anode of a tube at one resistor or the other - they're essentially in series.

They are in series for DC, but not for AC. That is the purpose of the decoupling cap. As Steve said, changing the plate load resistor will alter performance of the stage. REmember too that for a given B+ node, there might be four triodes - or more - running off it. All their currents add up. So altering the current through one triode will affect the voltage drop through the common resistor. The resistors beyond the decoupling cap are not part of the load.

Has anyone just dropped the V's with zeners? Sure would take a lot of guesswork out of things. I could rip out that power resistor, put in a couple suitable zeners and caps, and end up with EXACTLY the voltage I want at the heads of the anode resistors...

But the zeners need a certain amoint of current flowing to zene properly, won't they? That will likely be a lot more than your 7ma.

There is a relationship between your B+ and the mains voltage too. Roughly, for every volt of change in the mains, there will be about 4 volts change in the B+. A 5v change in the mains can mean a 20v change at the rectifier. And that is one reason there was the notation on the old Fender schematics that said: voltage readings may vary +/-20%. I wouldn't sweat the difference between 191v and 206v.

[gazinkus]

Thanks, Enzo

Unless this is a finely tuned machine, I might suggest worrying less about the voltage. It is real typical for voltages to be a lot higher in old amps now than they were back when, yet they still sound OK.

Well, I dunno about how "finely tuned" it is/will be, but it seems that if I'm shooting for a reasonable Plexi / Dumble imitation, it might be appropriate to emulate the anode voltages as well as the component values of the preamps. And the fact is, at the moment, I have too much oomph as compared to those designs. I need to reduce the oomph somehow.


They are in series for DC, but not for AC. That is the purpose of the decoupling cap. As Steve said, changing the plate load resistor will alter performance of the stage. REmember too that for a given B+ node, there might be four triodes - or more - running off it. All their currents add up. So altering the current through one triode will affect the voltage drop through the common resistor. The resistors beyond the decoupling cap are not part of the load.

Doh. It just dinged on me (I'm slow sometimes but eventually I catch on...). OK, so somehow I have to drop the B+ at the anode resistor head to something approximating the original design value, so that I can maintain the anode resistor value, therefore the gain structure of that original design.

But the zeners need a certain amoint of current flowing to zene properly, won't they? That will likely be a lot more than your 7ma.

Worse yet, since I'm likely to need 3 or so different B+ values for the preamps, each zener might only be handling 2mA or thereabouts. I didn't think about Iz.

There is a relationship between your B+ and the mains voltage too. Roughly, for every volt of change in the mains, there will be about 4 volts change in the B+. A 5v change in the mains can mean a 20v change at the rectifier. And that is one reason there was the notation on the old Fender schematics that said: voltage readings may vary +/-20%. I wouldn't sweat the difference between 191v and 206v.

I got the 191V value from a website showing the measured specs from a Marshall JTM45 (191 and 161, actually, at the anode, depending on the stage). I got the 206, 209, 227, and 229V values from the Dumble Clone website. I also have the power supply values for the Dumble Clone (323 and 361), but don't really have them for the Marshall. So, for the Dumble, anyway, it's easy to calculate the anode currents.

It's beginning to look like I need to redesign the anode power supply. Right now, I have a 445V B+ (for the audio tranny center tap) feeding a 4.7K and cap that supplies the phase splitter offset voltage. Then that feeds a 10K and cap for the anode supply. I measured 445, 440, and 410 at the three points with the old loading. So, I guess I need to figure out how much current is going out to the phase splitter. If I'm losing 5V across a 4.7K, that sounds like about 1mA to me. And that means I'm starting with 440V that I need to drop and filter for each of my anode supplies. I know I need 7.3mA total for the 8 preamp stages. So, to get to my first value of 361V (or thereabouts) that means... about 11K for a filter resistor. Right? Hmmm, that ain't all that different from what I have now (10K). Maybe I just leave that alone and branch off of it for the lower anode voltages I need. Comments?


By the way, thanks for the help.

Dennis

[Enzo]

Well, if those 8 stages are not powered off the same B+ node, then consider too that your 7ma is only at the power supply end, as each stage taps off the B+, then the current through the later node resistors is reduced by that much.

[gazinkus]

Yep

Well, if those 8 stages are not powered off the same B+ node, then consider too that your 7ma is only at the power supply end, as each stage taps off the B+, then the current through the later node resistors is reduced by that much.

Well, after considering your advice and contemplating upon it... and, ummm... borrowing... an idea from the Hoffman Amps version of a Plexi circuit with boost, I note that the Hoffman B+ supply has a 10K feeding a cap and the higher-voltage anodes, followed by another 10K and cap stage feeding the lower voltage anodes. If my calculations are correct, the first 10K - at the load I expect (and evidently the load that Hoffman expects) will draw the B+ down to around 365 (close enough to 361 that the 3rd and 4th stages of the Dumble Clone design wants, and close enough to what it looks like the 3rd and 4th stages of the Hoffman design wants), and another 10K will drop another 40 volts and get me down in the 325 range for the lower voltage first two stages of both preamps. So, essentially I end up duplicating what Hoffman did for the last two stages of their power supply and look to make both preamps have near the expected voltages.

Right now, I only have the first 10K and cap - Evidently Traynor liked to run all of their preamps hot. But given that the YVM-1 only had 2 stages - 4 paralleled 1st stages mixed by a 2nd stage, I guess it makes sense. If you want only have two stages, you might want all the oomph you can get.

Thanks again, Enzo...

Dennis

[Pentamonium]

Supplier of Electrolytic Capacitors?

[QUOTE=gazinkus;28407]
So you don't think the electrolytics alone could cause such a problem. Makes sense. Just to be safe, I have new electrolytics AND new tubes coming. I found a source (finally) for the 2-in-a-can electrolytics (JJ 50/50, 500v versus the Mallory 40/40 450v), ...

Where did you find those capacitors ?

Thanks
Stefan

[gazinkus]

Stefan:

I used Antique Electronic Supply, http://www.tubesandmore.com/ . Go to Capacitors, Multisection, JJ Electronic LCR. I think they're the 3rd item down. $8.75 ea (US) without the mounting rings.

I had to work pretty hard to get the old Mallory's removed from the Chassis, then drill holes for the mounting rings. Now, at a cursory glance anyway, it looks like the JJ's have been there all along.

Good luck,
Dennis

P.S. Yeah, it ended up being more than just the electrolytics. Turns out that several tubes were bad. The phase splitter tube would wait a couple minutes to warm up before it would start to flake out, and one of the EL34's was bad, as well. But everything else behaved once I changed all that out.

[Enzo]

If I want to remove a bad multican cap, and I don't want it to be saved for some odd reason, I often just cut the twist tabs off at the chassis with flush cutters, rather than trying to straighten them and melting solder so I can pull them out through the mounting slots. You can also just grab the twisters with pliers and work them back and forth until they break off.

[gazinkus]

If I want to remove a bad multican cap, and I don't want it to be saved for some odd reason, I often just cut the twist tabs off at the chassis with flush cutters, rather than trying to straighten them and melting solder so I can pull them out through the mounting slots. You can also just grab the twisters with pliers and work them back and forth until they break off.

In this particular case, the tabs were soldered to the inside of the chassis, pinning the can against the top side. I had a heck of a time getting enough heat applied to get the solder loose. I ended up cutting the tabs with a Dremel tool, freeing the can, then individually attacking the solder points with the biggest gun I had (350W). I didn't get things as clean as I would ordinarily like - but I got the tabs off and most of the surplus solder. Traynor must have used the Soldering Iron From Hell to put them on in the first place.

Dennis

[bentwire17]

Just p/up a 40 yr old couger in nice shape (YVM -1).
Original condition.
What mods do I want for a guitar guy looking fore nice blues/rock tone.
To Plexi or not to Plexi?
Clean and Quiet "What a concept"!!!!
Hector "Help Me"....
Regards ..in Port Perry ,on

[olddawg]

I don't know if anyone else mentioned it. If you still want to add another preamp tube and need more current you can usually replace the power indicator lamp with an led. Since the lamp was run off of the filament line it will give you a little more heater current available.