LTP Questions

[RadioComm]

Hello,

Just recently started re-troubleshooting an issue with my favorite amp, a 1979/1980 Fender Model 30. Out of sheer luck, I stumbled across the issue and fixed it. The discussion I had with @ac427v and @pdf64 was for me very educational. @ac427v had picked up on the fact that there was a discrepancy between the schematic (attached) and the actual circuitry. The delta was that the schematic had two 2000pf caps from pin 5 of the 6l6 to ground (which @ac427v explained that this was to cut high frequencies to prevent high frequency oscillation over time), and no anode to anode cap in the LTP. My Fender Model 30 though had the exact opposite. No PA tube grid to ground (g2g) caps, but had an anode to anode (a2a) cap. I did a little research after the discussion to establish whether the mod was a Fender Corporate decision or an individual's mod gone awry. All pictures I found pointed to a Fender decision, including this 100% stock picture I found (also attached). @pdf64 also stated:

Those LTP anode to anode caps aren’t used on any other other models that have a 12AT7 LTP.
The 12AT7 has a far lower anode impedance than the 12AX7 those caps are usually used with.

So I would consider this amp to have a very unique LTP. So here goes with the questions:
1) @ac427v did a great job explaining the 6l6's 2 g2g caps. But I'm still not clear on the what the intent of the a2a cap. What's its purpose? And why are they mutually exclusive with the g2g caps (or are they)?
2) One of the features of this amp that really thrills me, is the fact that the PA compresses at significantly much lower volumes than my other amps (including two mid-sixties fenders (bandmaster & bassman) and two early seventies fenders (TR & SR)). Could the uniqueness of the LTP (as noticed by @pdf4) contribute to this?
3) If not the above, what could have been the motivation for Fender to make that change? (besides using 1 cap instead of two to save on one cap and two minutes of manual labor, and don't laugh... From my own personal experience that's what management imposed on engineering for this one company that I worked for)

A very special thanks to @ac427v and @pdf64 and everyone else on this forum for making this forum the best.

Take care

[pdf64]

…
1) @ac427v did a great job explaining the 6l6's 2 g2g caps. But I'm still not clear on the what the intent of the a2a cap. What's its purpose? And why are they mutually exclusive with the g2g caps (or are they)?

The anode to anode LTP cap, and the output valve grid to cathode caps, are doing the same thing, ie rolling off high freqs.
The latter is probably more effective.
There’s no point doing both.

2) One of the features of this amp that really thrills me, is the fact that the PA compresses at significantly much lower volumes than my other amps (including two mid-sixties fenders (bandmaster & bassman) and two early seventies fenders (TR & SR)). Could the uniqueness of the LTP (as noticed by @pdf4) contribute to this?

I don’t see how 

…
3) If not the above, what could have been the motivation for Fender to make that change? (besides using 1 cap instead of two to save on one cap and two minutes of manual labor, and don't laugh... From my own personal experience that's what management imposed on engineering for this one company that I worked for)
…

Yes, cost saving seems a plausible explanation for the change.

[Willabe]

2) One of the features of this amp that really thrills me, is the fact that the PA compresses at significantly much lower volumes than my other amps (including two mid-sixties fenders (bandmaster & bassman) and two early seventies fenders (TR & SR)). Could the uniqueness of the LTP (as noticed by @pdf4) contribute to this?

It's probably the PT and OT set.

[RadioComm]

The anode to anode LTP cap, and the output valve grid to cathode caps, are doing the same thing, ie rolling off high freqs.
The latter is probably more effective.
There’s no point doing both.

Understood. Thanks @pdf64.

It's probably the PT and OT set.

Thanks @Willabe. So If I were to build an amp that compresses at lower volumes, what kind of specs would I look for in a PT/OT set?

[Willabe]

Thanks @Willabe. So If I were to build an amp that compresses at lower volumes, what kind of specs would I look for in a PT/OT set?

A little undersized rather than oversized.

Can't give you exact numbers, but it has to be within reason so as not to over heat the PT. I seem to remember PRR as saying it can be hard to burn up an under spect'd, power wise, OT? 

(Edit; Under size just the B+ on the PT, not the heaters.)

[pdf64]

….So If I were to build an amp that compresses at lower volumes, what kind of specs would I look for in a PT/OT set?

The HT supply needs to be saggy, so the PT HT winding shouldn’t be a low resistance type. But if it is, external resistors can be added.

And of course, a valve rectifier helps.

Likewise for the choke or dropper resistor between OT and screen grid nodes, ie more resistance will lend a saggier response.

What’s the red to red resistance of the 30’s PT HT winding? I hope it’s well over 100ohms, to help the 5U4 cope with the hot switching standby and massive 110uF reservoir capacitance.

The choke’s resistance is the typical about 120ohms?

https://el34world.com/charts/Schematics/files/Fender/Fender_30_schem.pdf

[RadioComm]

A little undersized rather than oversized.
Can't give you exact numbers, but it has to be within reason so as not to over heat the PT. I seem to remember PRR as saying it can be hard to burn up an under spect'd, power wise, OT?

Perfect answer. All I needed until next project.  Thanks.

The HT supply needs to be saggy, so the PT HT winding shouldn’t be a low resistance type. But if it is, external resistors can be added.
And of course, a valve rectifier helps.
Likewise for the choke or dropper resistor between OT and screen grid nodes, ie more resistance will lend a saggier response.
What’s the red to red resistance of the 30’s PT HT winding? I hope it’s well over 100ohms, to help the 5U4 cope with the hot switching standby and massive 110uF reservoir capacitance.
The choke’s resistance is the typical about 120ohms?

Thanks.
Pulled out the rectifier tube and measured across pin's 4 and 6. Got 50.4 ohms.
Don't have the amp up on the bench, but pull out spec sheet of replacement choke which states 167 ohms +/- 15%.  Don't know where mine falls within (or out) of the error margin. Links to Tubes and More replacement choke:

https://www.tubesandmore.com/products/choke-hammond-fender-deluxe-deluxe-reverb-etc
https://www.tubesandmore.com/sites/default/files/associated_files/p-c194a.pdf

Thanks again @pdf64.

[pdf64]

Pulled out the rectifier tube and measured across pin's 4 and 6. Got 50.4 ohms

That’s typical for Fenders, but will probably stress the 5U4, especially given the 110uF reservoir, and triple especially as standby hot switches it.
It would be best not to use standby.

Whatever, it seems most of your sag / power compression will be due to the 5U4.

[RadioComm]

That’s typical for Fenders, but will probably stress the 5U4, especially given the 110uF reservoir, and triple especially as standby hot switches it.
It would be best not to use standby.

Good to know. Thanks.

Whatever, it seems most of your sag / power compression will be due to the 5U4.

Agreed. Makes sense. I bought the amp back in the early to mid 90's, and originally the amp had a 5R4 in it. Maybe the previous owner's attempt at reducing the sag. Immediately after bringing the amp home, can't remember the reason (probably tone related), I yanked out the 5R4 and replaced it with a used 5U4GB that I had hanging around and had bought used at a hamfest for a couple of bucks.

Thanks again @pdf64.

Hope everyone has a great weekend!

[Willabe]

Just move the stand by switch so it's after the 1st B+ filter cap(s, if it's stacked). Works fine then.

Like this;

https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab763_schematic.pdf
More protection takes some stress off the rect. tube and will save your PT if the rect. tube shorts open. Just a pair of SS diodes in series before the rect. tube.

https://r.search.yahoo.com/_ylt=A2KLfSE.4RdhDDIAJjBXNyoA;_ylu=Y29sbwNiZjEEcG9zAzEEdnRpZAMEc2VjA3Ny/RV=2/RE=1628983743/RO=10/RU=https%3a%2f%2fwww.premierguitar.com%2fthe-immortal-amplifier-mod/RK=2/RS=LNDlj9mAdy_QFp..JAJLhU7DCoQ-

[RadioComm]

Just move the stand by switch so it's after the 1st B+ filter cap(s, if it's stacked). Works fine then.

Like this;

https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab763_schematic.pdf
More protection takes some stress off the rect. tube and will save your PT if the rect. tube shorts open. Just a pair of SS diodes in series before the rect. tube.

https://r.search.yahoo.com/_ylt=A2KLfSE.4RdhDDIAJjBXNyoA;_ylu=Y29sbwNiZjEEcG9zAzEEdnRpZAMEc2VjA3Ny/RV=2/RE=1628983743/RO=10/RU=https%3a%2f%2fwww.premierguitar.com%2fthe-immortal-amplifier-mod/RK=2/RS=LNDlj9mAdy_QFp..JAJLhU7DCoQ-

Thanks @Willabe. Really good mods. Probably won't do second suggested mod, but eventually may get around to doing the first. I get it, gambling with my PT, especially with a $2.00 used tube rectifier that I put in over twenty years ago. In the meanwhile, I have an old GT octal SS5U4 somewhere down in my basement, going to plug that one in (hopefully I can find it...), check out the difference in tone, and see if I still like the amp as much. Will proceed from there.
Thanks again.

[Willabe]

In the meanwhile, I have an old GT octal SS5U4 somewhere down in my basement, going to plug that one in (hopefully I can find it...), check out the difference in tone, and see if I still like the amp as much.

That SS rect. will raise the B+, have to check it that it's not too much for the power tubes AND if not to high B+ dcv, then rebias the power tubes. 

[RadioComm]

In the meanwhile, I have an old GT octal SS5U4 somewhere down in my basement, going to plug that one in (hopefully I can find it...), check out the difference in tone, and see if I still like the amp as much.

That SS rect. will raise the B+, have to check it that it's not too much for the power tubes AND if not to high B+ dcv, then rebias the power tubes.

Oh absolutely! Thanks.

[RadioComm]

Well plugged in the SS5U4. Re-biased. And as expected lost all of my beloved sag. Still a great amp, still my favorite, but want my sag back. Also as expected the delta with B+ was huge. Really had to crank that bias pot to get the bias back to where I wanted it.

Anyway, found my diode box, have more than a couple 1N4007's, plenty of shrink tubing. It's an easy mod, and certainly worthwhile to protect my PT. I have plenty of used/new/NOS rectifier tubes, so they're a little more expendable. So I know I won't be moving the standby switch up the filter cap chain. And of course, will re-bias after the mod and plugging back in that zillion year old 5U4GB (It's a Raytheon US Navy Stock).

Thanks again @Willabe for the mod. Added protection is always good.

Take care.

[RadioComm]

Added the 1N4007's, put back in the same 5U4GB, re-biased, and the amp is back to being awesome. This time though with the PT safe-guard. Quick and cheap addition. A lot of bang for the buck. Thanks again @Willabe and @pdf64 for pointing this out.

The fender 30 is a unique and extraordinary amp. Very different in design and approach to its sibling the Fender 75. If I had to guess, Fender's chief engineer Ed Jahns was responsible for the development of the Fender 75, and Bill Hughes undertook the Fender 30. The Fender 30's design approach is more aligned with Randy Smith's Mesa Princeton amps, which would be a hot-rodded Fender base amp. The Fender 30 includes such throw-backs for its time period, such as the tube rectifier and a "real" bias adjustment.

I've attached a picture of the diode rectifier safe-guard, and a pdf of RG Keen's article in Premier Guitar for convenience to anyone interested in this mod.

Take care, hope everyone has a great weekend.

[sluckey]

Most people move the red wires to pins 3 and 5, then mount one diode between pins 3 and 4 and the other diode between pins 5 and 6. This way the diodes are securely supported on each end with no 'flying' leads. And the diodes are clearly visible so no guessing about the heat shrink tubing.

[RadioComm]

Most people move the red wires to pins 3 and 5, then mount one diode between pins 3 and 4 and the other diode between pins 5 and 6. This way the diodes are securely supported on each end with no 'flying' leads. And the diodes are clearly visible so no guessing about the heat shrink tubing.

Uhg. Didn't think of that. What a great idea! Not in RG Keen's article... Just did it the way he suggested. Headed back down to the basement, got get this amp back up on the bench. Will post new picture later today. Thanks @sluckey.

[RadioComm]

There. So much better! Thanks again @sluckey.

[pdf64]

It’s worth noting that if ever called into service (ie due to the 5U4 shorting), such 1kV back up diodes should be fine for a Fender 30, but are somewhat underrated for higher voltage amps.
Calling for a 1kV type may have been an oversight in the original article 

[RadioComm]

It’s worth noting that if ever called into service (ie due to the 5U4 shorting), such 1kV back up diodes should be fine for a Fender 30, but are somewhat underrated for higher voltage amps.
Calling for a 1kV type may have been an oversight in the original article 

Noted. Thanks @pdf64

The article states the 1kV for up to 450V. Keen recommends for amps  in the range of 450-550V to use the "ST Semconductor STTH112U, which is rated for 1A and 1200V".

For some reason I was thinking B+ and not the red output of the PT. Fender 30 B+ in schematic states 415VDC, mine measures 388VDC.  Didn't think of measuring the red output of the PT.

So don't know either  , I think I should be good...

[RadioComm]

Well... Maybe not so good after all... Found the Hammond replacement PT spec (290cx) which I have attached. Looks like the red to red is 714.2V. Should I be using a higher rated diode?

The article references "B+ supplies"... Is that voltage measured at B+ or the PT red to red output?

If B+ junction is the answer, then I'm good. Fender 30 B+ in schematic states 415VDC, mine measures 388VDC.

Here is the exact quote from the article:

"The diodes have to be able to withstand at least twice the B+ voltage in the amplifier. For amps up to about 450V in normal operation, you can use the cheap and widely available 1N4007, which is rated for 1000V and 1 amp. For amps with B+ supplies over 450V up to about 550V, use the ST Semiconductor STTH112U, which is rated for 1A and 1200V"

[sluckey]

Quit pickin' at it! You're fine.

[RadioComm]

Quit pickin' at it! You're fine.

lol  Thanks sluckey

ttfn

[Willabe]

Quit pickin' at it! You're fine.

     

[tubeswell]

The article states the 1kV for up to 450V. Keen recommends for amps  in the range of 450-550V to use the "ST Semconductor STTH112U, which is rated for 1A and 1200V".

Diode PIV rating must exceed peak AC voltage. 1N4007 can withstand up to 714 Vrms, but Merlin reminds us also to make allowances for variation in mains voltage and transformer load to be safe, so HTmax = 580 Vrms in a FW bridge rectifier or 290 Vrms (i.e. 290-0-290) in a 2-phase rectifier. You can put them in series to increase overall PIV http://www.valvewizard.co.uk/bridge.html

[PRR]

...the red to red is 714.2V....

Which is 357V each side. Which will make almost, uh, actually, 504V of DC.

I'd still leave it alone. The "1000V" 1N4007 is almost certainly good for over 1200V; the days they had trouble meeting spec are far behind us.