Octal based Express

[10thTx]

FusionBear,

Thanks for the edits comments. I'll try to follow them the best that I can and redraw and repost sometime tomorrow Wed. May be later in the day.

With respect, 10thtx

[fusionbear]

Fifth, the cathodes of V3 are connected to a 75 ohm resistor.

FB,

Is that really a 75R resistor or 750R?

Thanks.

Yes, a 75R. 75 little Ohms. If anyone knows of any possible bad side effects of this value, let me know...

[fusionbear]

FusionBear,

Thanks for the edits comments. I'll try to follow them the best that I can and redraw and repost sometime tomorrow Wed. May be later in the day.

With respect, 10thtx

I am truly thankful to you!

I just noticed that the triodes are not labeled correctly.

V1a and V1b are parallel.

V2a is the tone stack recovery.

V2b is a gain stage

V3a and 3b are the Phase inverter

V4, V5 are the 6550A or 6L6GC power tubes.

[10thTx]

Deleted to not confuse my posted schematic with the current version by FusionBear

[martin manning]

Sixth, the PPIMV pot is wired backwards like a 5E3 so that the .1 couplers feed the wipers. This way there is always a 250k reference to ground through the pots.

It will work better the other way around. As it is now is you will lose some low end at low volume settings; exactly what you don't want.

Yes, a 75R. 75 little Ohms. If anyone knows of any possible bad side effects of this value, let me know...

The PI is biased hot, and that makes the cathode voltage high. 6SL7 is rated at 90V Vh-k, and I'm betting you are well beyond that. I suggest putting a 750 or 820 ohm there. How did you arrive at that value?

I don't think I would be calling this an Express, its pretty far from that now!

[10thTx]

The PI is biased hot, and that makes the cathode voltage high. 6SL7 is rated at 90V Vh-k, and I'm betting you are well beyond that. I suggest putting a 750 or 820 ohm there

I'm thinking a 750 ohm may be more useful also.

With respect, 10thtx

[M Fowler]

The tubes are labelled wrong should be 6SL7.

[10thTx]

The tubes are labelled wrong should be 6SL7

Yikes! Corrected.



Thank you! With respect, 10thtx

[fusionbear]

Hell Gents,

First, I want to say how much I appreciate your comments and suggestions. I will be the first one to admit that I have a lot to learn. I tried your suggestions and here is what I found:

1. Re-wiring the PPIMV so the wipers feed the power tubes sounded flat awful. All the dynamics were gone and the sound actually got much thinner. So, I am thinking that because it is cathode biased, the input to the power tubes need to see that 220K-250k reference to ground at all times. As soon as I re-wired back to the way I had it, the tone came back. Also, the 2.2M ohm resistors are not needed because the 250K reference is not dependent on the wipers like a typical LAR/MAR PPIMV, so I removed them. There is almost no Bass loss the way I had it...

2. I tried 820 on the PI cathode and it sounded too thin. 470 Ohm was perfect IMO, so that is what is there now instead of the 75 Ohm resistor. The reason I chose the 75 Ohm resistor is that is what I saw and measured in a Red Iron T-Rex. But, it definitely pushes the PI too hard. 470 Ohm it is...

3. I added a 47pf "fizz" cap on the plates of the PI and that smoothed out the highs without killing the tone.

I will post some pics of my work later. Thanks again gents!

[martin manning]

I'm very surprised that wiring the PPIMV in the "normal" way degraded the sound. I can't think of any reason for the 250k load being necessary in either fixed or cathode bias. Was it only at low volumes that the sound was bad? Surely it should have sounded exactly the same with the PPIMV all the way up.

[10thTx]

I'm very surprised that wiring the PPIMV in the "normal" way degraded the sound. I can't think of any reason for the 250k load being necessary in either fixed or cathode bias. Was it only at low volumes that the sound was bad? Surely it should have sounded exactly the same with the PPIMV all the way up.

I'm wondering the same thing? I think the 2.2M resistors are intended to be there as a "safety" thing incase on of the dual gang pots goes bad. So there is always some reference to ground?

Regarding the "enhance" cap vs. snubbing cap that you used. There is no reason to change it if you like what you're hearing.

Having said that, 100% of the time when I tried a snubbing cap (similar to what you've used), it sounded like I put a blanket over the speaker and killed the higher frequency tone.

I have removed a snubbing cap almost immediately every time I tried one. After playing just a few chords and riffs, the snubbing cap came out. This is with using even a 10p cap and I've tried a range from 10p -120p with similar disappointing results.

In contrast, the "enhance" cap (to my ears) seems to simply remove the hash/harsh higher frequencies and does not remove any of the highs that I can tell. It's like there are warbling oscillating higher frequencies that quit warbling and smooth out.

On several dumblish inspired amps I built, the enhance cap seem to allow some blooming to the notes where the sustain increased and allowed the notes to sort of open up or "bloom". My uneducated guess is that it stopped oscillations on the higher frequencies since oscillations can create a harsh sound and kill sustain. With oscillations gone, the sustain improved and sounded smoother, IMO.

I use an "enhance" cap on every LTPI build I do now. It has become a must have for me. I just use it on the entrance plate to the LTPI only. Typically 120p-220p has worked well for me.

Just an FYI & I am not suggesting that you remove the snubbing cap if you like it.

EDIT: Deleted posted schematic to not confuse with FusionBear's current one.

With respect, 10thtx

[fusionbear]

Oh, I see, when you said "enhance cap", I thought you meant "snub cap".

I will try it, I haven't tried it before, so I will give it a shot. Always wanting to improve.

On the PPIMV, the amp sounded horrible until I almost maxed the volume knob. The way I have it wired, it sounds great at all setting. The "safety" 2.2M's are needed if you feed the power tubes from the wiper in the event that the wiper connection fails in a negative biased amp. The way I have it wired, the reference to ground is always there via a 2 watt pot.

On the typical PPIMV, the end of the pot is negative bias, not actual ground. On a cathode biased amp, the end of the pot goes to ground. After thinking about it a bit, I suppose I could have put two 220k's in the place of the 2.2M's to get the same reference. But, I like the way it works, so I will leave it for now. Also, I added a 510K across the first filter cap.

Definitely intrigued by the "enhance cap" I will go try it before the Thanksgiving festivities begin...

[martin manning]

On the PPIMV, the amp sounded horrible until I almost maxed the volume knob. The way I have it wired, it sounds great at all setting...

...On the typical PPIMV, the end of the pot is negative bias, not actual ground. On a cathode biased amp, the end of the pot goes to ground.

In a fixed-bias amp the bias voltage is the same as ground. Seems like there has to be something else going on there. "Thin" and "horrible" sounds like maybe a parasitic oscillation is occuring.

[Littlewyan]

Interesting the PPIMV sounds better that way. Mine is currently wired the normal way and I must say it sounds bad even if its only turned down a tiny bit. I'll try rewiring it your way.

[fusionbear]

Interesting the PPIMV sounds better that way. Mine is currently wired the normal way and I must say it sounds bad even if its only turned down a tiny bit. I'll try rewiring it your way.

On most builds and repairs, the normal way sounds just fine. I wire them the normal way when clients ask for a PPIMV. 250k dual gang, 2.2M "safety" resistors. Once in a while I will have a client request the "Rich mod" PPIMV. Same goes for that.