PT and Rectification Question

[ericnuke]

I built one of the trinity 18watt amps recently, not from the kit just used the layout and schematic. I used an old PT I had with 270v HT taps instead of the 290v taps the trinity uses. So as a result all my voltages are lower, obviously, but the amp works, sounds pretty good, is real quiet and the bias looks good, I think.

My question then is, is this ok? Could I be hurting anything? Or is this just behaving like if I had a VVR installed on the whole amp and turned down slightly?

Also, i'm using an EZ81 rectifier and am considering a diode rec instead to increase the B+ and also tighten up the low end. How can I calculate the B+ increase from diode rectification and how would I design such a rec?

One last question then, the PT also has 340v HT taps. Could I use those with some zeners to lower the B+? I don't know anything about zeners but would that be to much voltage to drop?

Thanks a ton for any thoughts. Also, I love this forum.

[M Fowler]

I suggest you leave that amp just like it is. Many people use 275v secondary instead of 290v. That probably is a very good sounding amp at the lower voltages.

Mark

[Phil_S]

I'm with Mark on this one. Leave it alone. If you want to employ the 340v supply consider converting the amp to 6V6's -- I wouldn't even think about doing that as it changes the character of the amp considerably.

On the rectifier, first a picky point of clarification. The EZ811 is also a diode -- a tube diode, actually a pair providing full wave rectification. I know you mean solid state diodes.

The EZ81 provides about 1.3x and solid state provides about 1.4x. This might get you about 20v more. Maybe, maybe not. Is is worth doing? It's hard to say. There is a school of thought that says you can "fix it 'til it's broken."

The heck with voltage and what other amps sound like. The only important question is, do you like what you have? If so, anything you do to it may not improve it. With amps, more voltage isn't always better.

[M Fowler]

That's how other amps evolved

Switched v1 to EF86 and EZ81 to a GZ34.

Then as Phil said some one decided to use the full power 340v because that works nice for 6V6 or 6L6 types and so forth.

270v is a good range for an 18w amp.

Mark

[ericnuke]

Cool cool cool. Thanks so much guys. Yeah I'm pretty happy with it at the moment. I do want to do some things to tighten up the low end but I've got a whole list of simple cap/resistor changes that i hope can accomplish that.

And thanks for the clarification on the diode be it tube/solid state thing!

I still think I would like to, at least for curiosity sake, add a solid state rectifier. I've been tying to do my homework and read up on it but there are a number of types that I've found, bridge, half wave, full wave... and I'm still not sure what to use or how to make it I guess. And it's much easier for me to talk with real people about this stuff than reading through countless internet pages.

[martin manning]

What you have now is a full-wave rectifier, using the two diodes in the EZ81. They have a common cathode, at pin 3, and the anodes are pins 1 and 7. If you were to pull the EZ81, and solder in two 1N4007's from 1 to 3, and from 7 to 3, with the banded ends on pin 3, you'd have it.

[ericnuke]

Awesome! Thanks so much.

[Phil_S]

To tighten up the low end, raise the value of the cathode bypass cap on the power tubes. 18watt.com seems to be down lately and I don't know if it will ever rise again or I'd recommend a search there. If I recall, you can go up to 1000uf.

[Allynmey]

Eric, ez81's will vary a bit. I've built a bunch of 18 watter and they are a little farty on the bottom end to me. I have one with sil. rect. (tightens it up a bit). Raise the Plate caps a bit and raise the cathode bypas cap some.

[ericnuke]

What you have now is a full-wave rectifier, using the two diodes in the EZ81. They have a common cathode, at pin 3, and the anodes are pins 1 and 7. If you were to pull the EZ81, and solder in two 1N4007's from 1 to 3, and from 7 to 3, with the banded ends on pin 3, you'd have it.

I did this and it really made a big improvement! Much tighter and cleaner, not in a less gain way, but better palm muting and big low power chords. It did jump my voltages a big, about 20v or so, and I think that helped make the amp a little britter as well. Not sure if technically that works but to my ears I don't keep going to the treble knob and wishing it had an 11 or 12 setting. Thanks so much!

Next issue then. This amp has a master volume just on the TMB channel. Volume after first triode then another after the tone stack just before the PI input. The first volume really only sounds good at about 3o'clock and any further up it getts real farty and splattery sounding, like something is over compressed. Lame terminology I know. I'm assuming this means i'm driving the second gain stage too hard into ugly clipping? Not really sure what to do about that as I would like to be able to get all the gain I can out of that channel. Any thoughts?

[martin manning]

This amp has a master volume just on the TMB channel. Volume after first triode then another after the tone stack just before the PI input. The first volume really only sounds good at about 3o'clock and any further up it getts real farty and splattery sounding, like something is over compressed. Lame terminology I know. I'm assuming this means i'm driving the second gain stage too hard into ugly clipping? Not really sure what to do about that as I would like to be able to get all the gain I can out of that channel. Any thoughts?

What are the values of the B+, the plate load, and the cathode resistor on that stage following the first volume control? You will get the most headroom if it is center-biased, so you may be able to tweak that. Is there a schematic that you can post?

[ericnuke]

?[/quote]
What are the values of the B+, the plate load, and the cathode resistor on that stage following the first volume control? You will get the most headroom if it is center-biased, so you may be able to tweak that. Is there a schematic that you can post?[/quote]

B+ at all the nodes are 357v, 332v, 266, 252

Here are my tube pin voltages as well if that's helpful

V1
1-101v
3-1.2v
6-105.3v
8-1.1v
V2
1-129v
3-.9v
6-251v
7-129v
8-133v
V3
1-185v
2-38v
3-81v
6-181v
7-38v
8-81v
V4/5
2-.04v
3-10.83v
7-353v
9-330v

Here's the schematic.

[martin manning]

The value of R9 isn't really 820k, is it? It should be more like R5, 0.82k (820 Ohms). The correct value there will make a huge difference. BTW, the voltage table is a good idea, but it's really hard to read in that format.

[ericnuke]

Woops, R9 is 820 ohms. I'll fix the schematic. Also, I didn't look at the pin voltages after I posted but I had them in a much easier to read format. I guess the forum software bunched them all together. I'll fix that too.

[martin manning]

Raise the cathode resistor on V2A from 820 to 1k5 and see how that works.