Tweaking a Traynor YBA-1 - PPIMV and Screen Resitors

[dbishopbliss]

Its been over a year since I last posted. I had an accident last year where I cut a tendon in my right hand so I couldn't build anything for a while. Then work kicked in, etc. I still need to finish my AC15 but I received a Traynor YBA-1 that I want to tweak first because I think the changes will take less time than completing the Vox.


My amp is a later Traynor (1975 I think). The schematic can be found here (edit, the original link was wrong). The schematic can be found here. The tweaks I want to make should be fairly simple (I think), but I want to be sure:

• Replace 500K Linear volume pots with 1M audio
• Install screen grid resistors
• Install PPIMV

The first tweak is a no brainer. Just swap the pots.


The second tweak I am not sure about. I have read that there should be screen resistors and the YBA-1 doesn't have them. However, when I look at the schematic I see R27 is a 470R 10W resistor connected from Pin 4 of both output tubes to ground. Isn't this a screen resistor? Any reason to change it?


For the third tweak, I have read about others using the Ken Fisher/Bruce Collins style PPIMV with these amps. On page 5 of this doc the author suggest using 100K resistors. I'm assuming 1/2 watt is ok. Please let me know if I should be using a higher rating.

[shooter]

Isn't this a screen resistor? Any reason to change it?

It is, and I wouldn't unless it's flaky, browned out, etc.

[John]

I'm sure you know, but that R27 is not connected to ground, but to B+ at the positive end of the filter cap.

[dbishopbliss]

I'm sure you know, but that R27 is not connected to ground, but to B+ at the positive end of the filter cap.

Yep. Seem like I don't need to make that change. Maybe the confusion is because many amps seem to have the resistors on the socket between pins 1 and 4. Come to think of it... why aren't the resistors there? Or is that for amps that are not fixed bias?

[John]

I don't know why. I bet someone does though. 

[BetterOffShred]

http://www.valvewizard.co.uk/gridstopper.html
At the bottom Merlin talks about it a bit,  Where was the 470-10W wired up in your amp?  On the tubes itself? or was it wired up by the Filter caps?  I wonder if they just used a single resistor in a higher wattage rating and put it as close as they could to both tubes to save money? You'd think a 10W would be more expensive back then, maybe not!

El34's/6l6's have Grid 2 on pin 4, and No connection on pin 6, so that's why they string it across there, to keep it as close to the tube as possible. I guess pin1 in some 6L6's is hooked up to the shell, but on GC 1 is not hooked up either, so you may see the grid resistor strung from 1 to 4 with B+ on pin 1.   

Thanks for sharing that document where the fella went through his amp.  I read most of it, pretty good read with a lot of it applicable to numerous other amps from the same period!

-Brett

[sluckey]

Maybe the confusion is because many amps seem to have the resistors on the socket between pins 1 and 4. Come to think of it... why aren't the resistors there? Or is that for amps that are not fixed bias?

You cannot put a screen resistor on pin 1 of an EL34/6CA7 tube. Pin 1 is connected to the suppressor grid which will usually be connected to pin 8 (cathode). It is common to connect screen resistors between pin 4 and pin 6. Maybe that's what you were thinking?

According to your schematic, the two output tubes share a single 470Ω/10W screen resistor. Nothing wrong with that.

Does this amp work properly? If not, I highly recommend you fix it before you start "improving" it. The changes you want to do can easily be done after the amp is working properly.

[DummyLoad]

uses a common 470R (R27) G2 resistor for BOTH power tubes. a more common practice in euro made amps...and canadians are almost euro types. 


--pete

[dbishopbliss]

Does this amp work properly? If not, I highly recommend you fix it before you start "improving" it. The changes you want to do can easily be done after the amp is working properly.

It is REALLY REALLY LOUD. Other than that it works fine. This thing is louder than any other amp I have ever had and that is with the volume at 1.5. I'm comparing to a Vibrolux and a Bandmaster and a 1990's stereo Marshall rig. I heard these were loud but I cannot imagine putting this thing up to 10.


I may want to tweak this thing further to lower the output. I read somewhere about using zener diodes to lower the B+ or something. That will be further down the road.

[DummyLoad]

and that is with the volume at 1.5

that amp has a very reasonable 400V B+. lowering the B+ with zeners would be a waste of time.

it's very similar to a 5F6A bassman circuit only the CF is configured as a gain stage with a bridged t-filter in front of the grid of V2B, so it will drive to full volume with not much turn on the dial + the volume pots are linear taper - swap those out with audio taper pots to help with that.


--pete

[dbishopbliss]

and that is with the volume at 1.5

that amp has a very reasonable 400V B+. lowering the B+ with zeners would be a waste of time.

it's very similar to a 5F6A bassman circuit only the CF is configured as a gain stage with a bridged t-filter in front of the grid of V2B, so it will drive to full volume with not much turn on the dial + the volume pots are linear taper - swap those out with audio taper pots to help with that.


--pete

That doc I linked to said something about the voltage being 480VDC and converting to a tube rectifier dropped it to 453VDC which is still higher than the Bassman according to the schematic.  That is why I was thinking of doing something to lower the voltage, but as I said that will come later.

[sluckey]

It is what it is. Maybe you should shop for a Bassman.   

[dbishopbliss]

It is what it is. Maybe you should shop for a Bassman.   

But this was soooo cheap.

[BetterOffShred]

It is what it is. Maybe you should shop for a Bassman.   

But this was soooo cheap.

I got my Bassman for $250 .. just saying.. but it's crazy loud too.  I really think you've got something serviceable there, and I'd probably really enjoy messing with it as well. I think like Pete said, throw an audio taper pot at the volume and go from there.  50ish watts is just loud. It really is.  I've played my Bassman dimed a handful of times, and yeah it's amazing, but it's always going to be loud.  That's why so many dudes are building 12w stuff haha! 
-Brett

[dbishopbliss]

I tried the "Trainwreck Type 3" master volume. Super easy to clip in and it worked... sort of. When I turned the volume all the way down I got no sound. If I snuck up the volume just a hair, then I could play at conversation levels. However, if I went anything above a hair, it was pretty much full volume again. I'm not sure if that is expected but that was my experience so I think I want to try the Rich Mod PPIMV.


The question I have is that the phase inverter for my Traynor has different values than Fender, Marshall and even earlier Traynor amps. I have included the schematic of the Phase Inverter below. Please let me know if the following:


Can I install the MV using the values most people are using? (500K dual pot)?


Should I consider changing the 68K resistors on my amp back to 220K resistors more commonly found?


Thanks for your help.

[chocopower]

In the traynor schematic on the first post there is a mistake.


As you said, pin 1 in one tube is linked to pin 4 in the other tube. And the other tube, too.
So. In one tube pin 1 is grounded and 4 goes to b+ (ok), but in the other, pin 4 is grounded a pin 1 goes to b+
 

[dbishopbliss]

In the traynor schematic on the first post there is a mistake.

I didn't create the schematic in the first post and it is actually different than my amp.  Based on the serial number, my amp is from 1976. The schematic on the bottom of the lid says that it was updated October 1975. You can find the schematic for my amp here. I believe that my scan of the schematic is the only one for 1975 online.

Any thoughts on the 68K resistors in place of the 220K more commonly found. There are also 1.5K resistors on the grid of the output tube which other amps don't seem to have.

[plumcrazyfx]

On these Traynors I usually replace the screen resistor and then add one on each of the tube sockets.  I replace the 10w because if you look at pictures of these there is frequently a nice burn mark on the schematic inside the top lid.  These things get hot and sometimes just crumble when you touch them.  Next on my list for an update is a 76 YBA-4 that's seen better days.  Traynors are super amps but they were working with really good tubes when they built them and you often see them needing work when someone just tries to throw any old glass in them.  I love my YGM-3 but that thing has 450v on the plate of the EL84s.

[Tony Bones]

Any thoughts on the 68K resistors in place of the 220K more commonly found. There are also 1.5K resistors on the grid of the output tube which other amps don't seem to have.

The 1.5k grid stoppers are fine, but yes replace the 68k resistors with 220k. Also replace the 47k resistors in the PI (R19 and R20) with 470k. It's a mystery why Pete Traynor put those very low value resistors in both places, but nobody ever regretted fixing them as far as I know.

[PRR]

Read the spec sheet for the power tubes. *Usually* the maximum safe grid resistor in fix-bias is 100K. 68K is conservative. 220K is brave. It "often works" in guitar amps because we bias pretty cold, so a little bias drift does not get into trouble; also because only a small % of tubes are as leaky/gassy as the max grid resistor spec implies.

[dbishopbliss]

Read the spec sheet for the power tubes. *Usually* the maximum safe grid resistor in fix-bias is 100K. 68K is conservative. 220K is brave. It "often works" in guitar amps because we bias pretty cold, so a little bias drift does not get into trouble; also because only a small % of tubes are as leaky/gassy as the max grid resistor spec implies.

I found a brief mention of the grid resistors in Richard Kuehnel's book "The Fender Bassman 5F6-A" on page 229. He says,


"There are other ways to affect bass respone and bias excursion dynamics, most notably by varying the value of the grid resistor Rg. This affects midrange gain, however and Marshall's grid resistor values are typically 220K..."


However, he doesn't say "how" it affects midrange gain. Do lower values have lower gain and higher values have higher gain?

[PRR]

> lower values have lower gain and higher values have higher gain?

In this (and many) case, yes.

But the difference of gain is not large. The risk of power tube run-away with large resistors is slim but very real. There are less risky ways to trim overall amplifier gain. I would not advise re-rigging 220K to <100K just because it is less risky. But if an amplifier is known to be happy with 68K, I would hesitate to up that to 220K just because everybody else did.

[dbishopbliss]

I'm back to thinking about the Trainwreck Type 3 style master volume. I played around with it some more and I found that putting the pot around 1/4 turn I was able to play comfortably. I could turn the volume controls down and play clean, but turn all the way up without going deaf. I measured the value of the pot and it was 50K.


So here is what I was thinking... Is there any downside to wiring a 50K resistor across the out of phase signals instead of the pot? Downside other than its a fixed value. Could this be dangerous in any way? If not, why don't more people do this to attenuate their amps? It still sounds good to me.


Thanks for your input.

[sluckey]

There's no harm in using a fixed 50K resistor in place of the pot if that's what you like. Heck, silvertone replaced that pot with a switch and called it standby. Works fine for a mute switch.

I bet that as soon as you replace the pot with a fixed resistor you'll find a situation where you wish you had the pot back. 

I never liked that crossline MV. Lots of others feel the same.

[dbishopbliss]

Update... while trying to clip in the master volume for more testing the end of the 1.5K grid stopper  snapped off. 


So I decided to repurpose the 1.5K cathode resistor used on the bright channel and replace it with a 820R resistor I already had. I believe this is the same value Marshall used in the 1987 plexi. The marshall also bypassed the cathode with a 0.68uF capacitor... I didn't have that value lying around, but I had a 1.0uF from an old Conn organ. The two channels sound very different now. I think the cathode bypass cap eliminates the need for the bright cap on the channel. I'm not a big bright fan.  Either that or I will install a switched volume pot so I can take it in and out of the circuit.


The other thing I did was install a 47K resistor between Pin 5 of the output tubes. I brought the amp to the northern virginia tone fest yesterday (check out The Gear Page for more info on that). I didn't mention the hard-wired master volume on 2. I got lots of positive feedback about how good and marshally it sounded. I will be recording some new clips soon.

[dbishopbliss]

I'm ready to move beyond the "fixed resistor" Type 3 control I put in. That is, I put a 47K resistor between Pin 5 of the output tubes instead of using a pot.


There are so many different master volume approaches out there it can get confusing... I see the Fisher, LarMar, Rich talked about a lot. Then I came across the Frondelli Master Volume on Rob Robinette's site. I think it is the same as Type C in this post. It seems like a really simple master volume to install. Rob lists several reasons why it is his "favorite master volume" including:

• The power tube grid leak resistance does not change when the master volume is adjusted which keeps the bias voltage from changing.
• No fail safe resistors are needed on the wiper like on the Type-2/Lar-Mar master volume because the wiper is used for signal input, not output to the power tubes. Bias voltage will not be interrupted if the pot wiper fails.
• The Frondelli master volume simply replaces the grid leak resistors with a dual-gang audio pot (one shaft turns two pots). For amps with 220k grid leaks use a 250k pot and for amps with 100k grid leaks use a 100k pot.
• With the master volume set to maximum the power amp circuit is just like factory. There's no added coupling capacitors to change the amp's voicing.

Sounds like it has advantages. Any thoughts why this approach isn't used more often?


On a side note. The Traynor uses 68K grid leak resistors. Oddly enough, I measured the both resistors the other day and they had a value of around 45K. The stripes indicate they should be 68K, but the values of both resistors are lower. Not sure why they would both measure so low when I measuring across the resistor.

[Ed_Chambley]

Why Dont you install jj 6V6S power tubes? They will handle the voltage and cut your wattage. If you cathode bias as you could also use a VVR easier.
Maybe even use speakers with a low SPL.


You could get your wattage less than half provided you maintain voltages of 450 or less. If you want more voltage drop, install a 10 watt dropping resistor or either a zener on the CT.


I did this to one of my Fender Super Reverbs and have a separate cab with 2 10s running a pair of 96db speakers. I have no master volume as I do not need one. When I do this I leave the old bias circuit and when I want to use the 4 speaker cab, which has 2 weber alnicos and 2 ceramic, all I have to do is connect a wire, swap cabs and put the 6L6gc back.


I have had a lot of guys ask me to do this, but the speaker in this case have to have a 4 ohm load, not 2. You can simply disconnect 2 in a super reverb. Warehouse speaker has a killer 10 cheap and you can order lower efficiency voice coils if you want to wait for them to make a run.


My Super Deluxe with 2 10s actually gets a better tone at lower volume than my Princeton, but my Princeton is 2 10s as well, 2 jbl with 100 SPL.


This works on 4 tube output amps even better simply because like a Marshall Super Lead. 100 watt with 4 EL34, 50 watts with 2. 34 watts with 4 6V6 cathode bias and 18 watts with 2.


If you are willing to add a large zener and/or a dropping resistor, you can use regular 6V6 tubes. Personally, I mostly use 7C5 since I can get RCA of these and I prefer RCA in Fenders.


Traynor voltages are usually on the high level. Still it can be done fairly easily.

[dbishopbliss]

Why Dont you install jj 6V6S power tubes? They will handle the voltage and cut your wattage. If you cathode bias as you could also use a VVR easier.
Maybe even use speakers with a low SPL.

You could get your wattage less than half provided you maintain voltages of 450 or less. If you want more voltage drop, install a 10 watt dropping resistor or either a zener on the CT.

I wasn't aware of the 6V6S tubes. I will investigate those but I still think I want to install a master volume.

[dbishopbliss]

More updates...


I changed the cathode resistor and bypass cap of the bright channel to Marshall 1987 values of 2.7K and 0.68uF respectively.  Sounded better than just bypassing with the 1.0uF cap I added before.


Next I changed the mixing resistors from 100K to 270K and bypassed the bright channel mix resistor with a 500pF cap. This is the best change yet!!! Now the channels sound very distinct. The normal channel is great for single coils and brighter guitars. The bright channel works great with humbuckers, filtertrons, and P90s. 


The last change I made was updating the Grid Leak and Tail resistor values of the Phase Inverter to match those on the Bassman/JTM 45. That is, I changed the Grid Leak Resistors from 47K to 1M and the Tail Resistor from 6.8K to 10K. I'm not sure if I am remembering the impact to the sound correctly (this is why I should have recorded between each change). But I think that there is much less headroom and the amp breaks up much earlier now.  Does that make sense?


If I wanted to reduce the gain should I change the Grid Leak Resistors, the Tail Resistor or both? Or, I am just imagining that the PI changes increased my gain?