HRM odd Layout - advice needed

[fabiomayo]

Hi, everyone.

I'm rebuilding my HRM originally built by a local tech. The PT is gone and
the builder kindly offered to replace it (it's been more than a year, his
stated warranty). I decided to rebuild it from scratch, try to make it
cleaner and simpler, and hopefully more 'dumbly' in sound. I'm taking
off tube rectification, triode/pentode switch, putting the post-OD TS back
inside the amp, and taking off a bunch of the preamp switch.

I was hoping y'all could take a look at my layout (thanks Normster
for the Visio and inspiration and Bob-I for your layouts too). It's my
second amp build and I'm very newbie so any help is much appreciated.

Please note that this amp has an odd layout and I was hoping not to
machine the chassis much (at all if possible, except for drilling the
occasional hole). Lots of parts are left unused and if I decide to do a
faceplate they'll go away.

Points to note:
1- Cathode resistors mounted on the board. Is that a problem?
2- Why not ground the OD channel?
3- PAB and channel switching may be wrong. Please take a look.
4- I've prepared to use the 5v secondary to power the relays, but I'm
gonna build without them first.



Thanks everyone. Circuit is HiGain's as I haven't had the time to go
through my notes (basically, advice collected here and from dogears)

All the best

[Bob-I]

Please note that this amp has an odd layout and I was hoping not to
machine the chassis much (at all if possible, except for drilling the
occasional hole). Lots of parts are left unused and if I decide to do a
faceplate they'll go away.

Points to note:
1- Cathode resistors mounted on the board. Is that a problem?

No, these are commonly mounted on the board. Your layout doesn't show them on the board though, which is also fine if it helps with the lead dress, but I always mount the cathode resistors and caps on the board.

2- Why not ground the OD channel?

This confuses me. What do you mean?

3- PAB and channel switching may be wrong. Please take a look.

No that looks correct to me.

4- I've prepared to use the 5v secondary to power the relays, but I'm gonna build without them first.

Yes, I've done that before too.

Looks like a workable layout.

[fabiomayo]

1- Cathode resistors mounted on the board. Is that a problem?

No, these are commonly mounted on the board. Your layout doesn't show them on the board though, which is also fine if it helps with the lead dress, but I always mount the cathode resistors and caps on the board.

Sorry... I meant grid resistors V1b (220k bypassed by the 500pF)
and V2a 68k. The others are mounted directed on the sockets as some
layouts here.

2- Why not ground the OD channel?

This confuses me. What do you mean?

I read somewhere on this forum that you shouldn't ground the OD when
in clean mode, but most schematics do. Should I lift the OD section and
ground it or not? If so, where? Together with the preamp ground bus?

Thanks, Bob-I. Your help is much appreciated. I got your 5v supply
schematic and since I gave up the tube rectifier it was pretty obvious
what to do...

How's the lead dress? Thanks to my poor understanding I'm always
worried about wires crossing each other or running too close parallel
to each other...

I have a couple more questions though...

1- Originally this amp had the 3-way ground switch with a .047uF (or
.47uF - I don't recall ). Why is it there? What is the purpose?
Is it for plugging into two conductor outlets? These are the most
common around here...

2- Should I keep a B+ fuse?

3- I was thinking of using a resistor instead of the choke. Is it useful
to have it switchable?


Thank you SO MUCH!

All the best.

[fabiomayo]

Ohhh... One more

How about the cathode resistors and bypass caps mounted on the tube
sockets and grounded on a separate ground bus? That's how the amp
came but I can change it if there's any potential for problems...

[Tdale]

Is the presence pot wired correct?

On the schem, there is a cap between the side-lug and ground, and the resistor connects to ground, without touching that lug.

On your layout, both the cap and resistor meet at that lug, before it's grounded.

When the wiper on the schematic is on top, the pot is bypassed, so the signal is divided between the 390R and the cap. That's how it is in your design too.

But when the wiper on the schematic is at the bottom, The signal goes through the whole pot, and then through the cap, while on your design, the cap is bypassed in that position...

Or am I wrong?

[dogears]

Runnin' the gain up a bit with an 8k feedback resistor! That may be way too high. I'd go back to the 4.7k

Also, a 50 watt amp might perform better with 50uf total filtration. In addition, you can remove the 470k that is bypassing the single filters. You only use them to equalize the series pairs.

Good luck!

[fabiomayo]

Runnin' the gain up a bit with an 8k feedback resistor! That may be way too high. I'd go back to the 4.7k

Ok. Thanks.

Also, a 50 watt amp might perform better with 50uf total filtration.

On B+1, is that it? Ok.

In addition, you can remove the 470k that is bypassing the single filters. You only use them to equalize the series pairs.

Thanks. I had no idea of what was the purpose of these resistors.

Is the presence pot wired correct?

Thanks for catching that. I don't even know why I did it that way
Fixed the file.

Thanks so much, guys. Really apreciated.

All the best.

[Tdale]

What program do you use to make the layout? It's beautiful!

Tommy

[fabiomayo]

What program do you use to make the layout? It's beautiful!

Tommy

It is Normster work, not mine. I just rearranged the parts to my schem
and layout. Program is MS Visio.

Attached is the visio file for anyone interested.

All the best and thanks for the advice.

[fabiomayo]

Okay, people. I went all the way and drawn the relays according to Bob-I's
schematic (the one with the 'virtual' ground). Please, take a look at the lead
dress and relay function. I added three relays: PAB, MidBoost and Drive.

As far as I can tell, with the front panel switches off, the footswitches should
work fine. On the footswitch, I'll have three switches: Drive+PAB, PAB alone
and MidBoost. Only way to access Drive without PAB would be through the
front panel.


Thank you all.

All the best.

[tictac]

That OT secondary ground should be grounded at the PI ground point not on the chassis so close to the input ground.

Also you may have noise problems with the OT mounted above the preamp like that. I have an amp with the OT mounted there and was picking up noise until I made a metal shield to cover the area around the preamp where the OT was.

TT

[fabiomayo]

That OT secondary ground should be grounded at the PI ground point not on the chassis so close to the input ground.

Also you may have noise problems with the OT mounted above the preamp like that. I have an amp with the OT mounted there and was picking up noise until I made a metal shield to cover the area around the preamp where the OT was.

TT

Thanks, TT. That's the kind of advice I need. I can run the OT ground over
to the PT ground. Is that okay?

This amp is a rebuild and I'm trying not to have another chassis made. This
is the way the trannies are mounted. Before hand, the amp was quiet. Not
dead quiet, but quiet. Since I'm cutting short all leads, I'm hoping to minimize
noise evne further. What do you think?


All the best. (I'll fix the file tonight)

[tictac]

Ground the OT where the 390 ohm PI tail resistor is grounded.

Neat lead dress is always a good idea. shorten and shield those grid wires where ever posible.

TT

[fabiomayo]

As I wait for the new PT (and the rest of the parts missing), here's some
changes. I cluttered the layout even more. The point being to exclude
the back panel and cut the chassis (of course, rebuild a wooden back
board) - doing so would make working on the amp sooooo much easier.
What do you think? Here's a pic of the amp as is and the new layout.

I also added a 'simulated' heater from lamp to V1 only but you get the idea

Any critics is deeply appreciated.

[fabiomayo]

Oh. I also plan on making a bigger back panel covering the tubes but
with an added oval cut and see how it sounds.