Rebuild. 5e3 preamp + 5f1 output stage.

[utervo]

Decided to do a complete rebuilt on my amp. I will reuse PT, Hammond OT and Hammond chassis and other reusable parts.

Hopefully mating of 5e3 preamp and 5f1 output stage with the modifications will bring enough brightness Eminence Cannabis Rex.

Input is very welcome, hopefully I've understood the schematics correctly and joined them at the correct point. The extra triode from PI is used as in 5F1, hopefully gain won't be too much?

The chassis is fairly big and I've contemplated on adding a 6BM8 reverb as someone has been doing music-electronis-forum but I don't know which would be the best place to take and input the signal.

[xtian]

Have you done the variable NFB before? I was disappointed with having a pot there. Even with a linear pot, very poor sweep. Instead a SPDT with three NFB choices was much better. The switch make a small pop in output, not bad.

[rock_mumbles]

You need a grid load (reference) resistor on the last 12AX7 stage between the 0,01uf cap and the grid

[utervo]

You need a grid load (reference) resistor on the last 12AX7 stage between the 0,01uf cap and the grid

Thanks, I added a 470k there now, is it appropriate value?

[utervo]

Have you done the variable NFB before? I was disappointed with having a pot there. Even with a linear pot, very poor sweep. Instead a SPDT with three NFB choices was much better. The switch make a small pop in output, not bad.

No I haven't done, what amp and value pot did you try?

[utervo]

As the schematic is now, how much gain there would be, not too much for the 6v6gt?

How woulda cathode follower instead sound as the last triode? More compression, more like 5e3?

[xtian]

Have you done the variable NFB before? I was disappointed with having a pot there. Even with a linear pot, very poor sweep. Instead a SPDT with three NFB choices was much better. The switch make a small pop in output, not bad.

No I haven't done, what amp and value pot did you try?

Details here: http://ampgarage.com/forum/viewtopic.ph ... 2&start=30

[utervo]

Here is the 6BM8 schematic I have been looking for.

Where would you position the reverb circuit input and output?

Any modifications needed to the schematic?

I don't understand if this schematic offers total bypass for the reverb, anyone?`

[Firestorm]

As the schematic is now, how much gain there would be, not too much for the 6v6gt?

How woulda cathode follower instead sound as the last triode? More compression, more like 5e3?

How much gain is "too much?" But you've got three cascaded triodes with only a tweed tone control (not very lossy) so you'll "never" be able to turn the volume up full. The gain will be even more than in an Express. I don't think a CF buys you anything in that position (and would screw up the conventional NFB input).

[rock_mumbles]

You need a grid load (reference) resistor on the last 12AX7 stage between the 0,01uf cap and the grid

Thanks, I added a 470k there now, is it appropriate value?

That's something that you may have to play with ... if that triode is getting too much input signal you'll need to reduce the value.

[Lynxtrap]

Here is the 6BM8 schematic I have been looking for.

Where would you position the reverb circuit input and output?

Any modifications needed to the schematic?

I don't understand if this schematic offers total bypass for the reverb, anyone?`

As far as I understand, you now have one gain stage less than in the schematic you posted first? Not that it will matter much, I guess. Three cascaded triodes seem a bit over the top in that circuit.

If by total bypass you mean completely out of the circuit, I think you'd need to be able to break the input line to the reverb somewhere, like with a switch?

[utervo]

How much gain is "too much?" But you've got three cascaded triodes with only a tweed tone control (not very lossy) so you'll "never" be able to turn the volume up full. The gain will be even more than in an Express. I don't think a CF buys you anything in that position (and would screw up the conventional NFB input).

Well it's not good use of the triode then if it's that much. I must rethink a bit.

As far as I understand, you now have one gain stage less than in the schematic you posted first? Not that it will matter much, I guess. Three cascaded triodes seem a bit over the top in that circuit.

The second schematic is a 5F1 with a 6BM8 reverb.

I am planning to use the 5E3 input section and use the 5F1 output section as that is what the iron is for.

If by total bypass you mean completely out of the circuit, I think you'd need to be able to break the input line to the reverb somewhere, like with a switch?

Yes that's what I thought, I wonder if I need to break the output section at the same time. Will it pop or be ok with just a switch?

[Lynxtrap]

Yes that's what I thought, I wonder if I need to break the output section at the same time. Will it pop or be ok with just a switch?

Hold on until you get somebody else's opinion as well. I'm not sure about it.
But as to popping, I imagine it could potentially make a terrible capoosh

Using lower gain tubes in the preamp would of course tame the signal a bit.

I have no experience from that particular feedback circuit. But I wonder what would happen if you use the pot in the feedback and went really low on the resistor, like less than 1K, or even leave it out completely? The pot might then work as a kind of master volume control and control the amount of gain in combination with the ordinary volume pot.
I'm just throwing this out as ideas, don't take is as fact.

Of course there are other ways of taming the preamp. In this kind of SE amp, many prefer to have the output stage sweating, not having the preamp clip too much.

[surfsup]

There is a footswitch grounding the second side of the reverb out, coming from the pan just after the return

[utervo]

Here is a layout sketch. Decided to not do the reverb as I fear it will complicate things too much.

Wires missing and no grounding planned yet. Things are in a bit odd places to cover all the holes in the chassis.

Lots of switchable bypass cap options. Added film bypass caps parallel that are always on in case it would help to retain highs that the 5F1 was missing. Any problem paralleling like that that?

Is it ok to place the 100k plate resistors head to head like I have done, any experience?

Does B+ get easily contaminated by AC hum?

The filter caps are quite close to the power transformer and at the back there is AC wires going next to B+ as the VVR will be there.

How do I calculate correct fuses now that there is an extra bottle? I want to add B+ fuse too.