Cool old amp. I have never been able to get any info on it...

[jbow]

Yep, I had a Sound Projects amp that was exactly like that: heaters fed from the AC line, but a power transformer for B+, and I did not see a need for an isolation transformer.

It would be great if the OP could sketch us a layout showing the wire connections, then we can figure it out.

I will try to get it done next week.... I have some other things to do and I have never done this before, so I may need some help. I don't know what all the components are but will ask about what I don't know. I've tried to learn tube electronics and still struggle with schematics... it just has not clicked for me yet. I have built several amps from wiring diagrams that work well. I went to the local technical school to try and learn but it was a total waste of time. I've several good books on tube electronics and tube audio design, still I struggle with schematics.

I appreciate the help and the patience.

Julien

[nashvillebill]

You can make a layout at first, like the second page of this Harvard: http://www.webphix.com/schematic%20h...5f10_schem.pdf

The two dashed lines going all the way from left to right show how Fender folded the sheet metal.

[jbow]

Thanks.. yeah, I understand a layout. I've built several 5f1 clones and one 5e3, they all work well except for a fourth 5f1 that I cannot figure out what is wrong. My only problem with this Stewart will be figuring out what a few of the components are. There are a couple of components that could be resistors or caps, I cannot tell. They certainly aren't carbon Comps, nor are they paper caps. I'll ask about them when I get to it... these days everything seems to be a chore for me. Bad back, neck, arthritis, headaches, no motivation... but one has to keep going... I am just a bit slower now... especially when a cold front is about to move through. Sorry... would you like some cheese with the whine? LOL.
Thanks for the tip about the dotted lines, I was thinking about how to draw that.
You guys are really helpful, I appreciate it. I wont proceed until I get a layout done, it make be a week or two but I will get to it. I have to record a guitar part this week and that will take up more time than it should...

Thanks,

Julien

[cthulhu]

Hi Julien,
of course no matter what else you do you'll want to replace those electrolytics.

[jbow]

Hey Cthulhu! I will. One of the paper caps... the outer paper turns around the inner paper. I haven't given up on it, I've just had to set it aside again for a bit. Maybe I can get the layout done soon. I have to study for two big exams for new licenses for my business and I just got a Mesa Boogie Mark V combo that I am learning (WHAT AN AMP!!) Life is god though. are you "the" Cthulhu... from the Diner and the old HCGF etc?
I wonder if anyone around here ever sees Bob Clear aka Admiral Ballsy. He seems to have fallen off the world...

Julien

[cthulhu]

Hey Cthulhu! I will. One of the paper caps... the outer paper turns around the inner paper. I haven't given up on it, I've just had to set it aside again for a bit. Maybe I can get the layout done soon. I have to study for two big exams for new licenses for my business and I just got a Mesa Boogie Mark V combo that I am learning (WHAT AN AMP!!) Life is god though. are you "the" Cthulhu... from the Diner and the old HCGF etc?
I wonder if anyone around here ever sees Bob Clear aka Admiral Ballsy. He seems to have fallen off the world...

Julien

Hi Julien,
Yes that is me! And yes I have been in touch with Bob Clear; in fact he just bought a guitar from me.

[trobbins]

We don't have the benefit of reviewing the complete schematic of this amp. However, if it is just the tube heaters that are in series and connected directly to the AC line AND the B+ supply is derived from the secondary of a transformer AND a grounded power cord is properly fitted to the amp...Then I don't see the need to use an isolation transformer with the amp. After all, we don't feel the need to add an isolation transformer to a table lamp or our Christmas tree lights. Right?
Comments invited.
Tom

My hassle with that are the suspect creepage and clearance issues:
1: Thickness and quality of solid insulation between heaters and cathodes relates to all valves installed.
2: Many adjacent terminals on valve bases, with varying degrees of pollution.

And then there is the issue of servicing the amp with power on, and the likelihood of mains voltage being significantly above the original design intent.

Ciao, Tim

[Enzo]

But an isolation transformer does not address those concerns.

An iso is 1:1, so if the mains are higher, then so will be the isolated mains fed to the amp. The thickness and propinquity of heaters and cathodes doesn't change, they get the same 120v or whatever from the iso as they do from the wall. Servicing the amp while it is powered? Name me one amp that is mains powered but has no mains connections inside?

[trobbins]

My concern is related to the breakdown of the heater-cathode solid insulation and exposing the circuitry to mains primary (rather than a Tx secondary). A higher working voltage across the insulation increases that risk.

Wrt to servicing, the mains primary is typically restricted to a few terminals relating to fuse, switch and PT primary taps - often co-located away from valve sockets, sometimes covered, and sometimes marked. In contrast, every valve socket would have two uncovered and unmarked mains primary terminals.

[J M Fahey]

We don't have the benefit of reviewing the complete schematic of this amp. However, if it is just the tube heaters that are in series and connected directly to the AC line AND the B+ supply is derived from the secondary of a transformer AND a grounded power cord is properly fitted to the amp...Then I don't see the need to use an isolation transformer with the amp. After all, we don't feel the need to add an isolation transformer to a table lamp or our Christmas tree lights. Right?
Comments invited.
Tom

You don't plug a guitar into your table lamp or Christmas tree lights, which means you are firmly grabbing with your naked (and often sweaty) hand: strings, bridge, etc, directly connected with a copper wire to dubious chassis ground.
I wouldn't trust my life to the hundredths inch separation between cathodes (connected to said chassis i.e. your hands) and *EASY* to bend red hot wires, suspended in vacuum.
Just saying.

EDIT: and trobbins points to the main point: live voltage is on each and every tube socket.
Which will make modding or even servicing .... "interesting" ....

[Enzo]

I am not advocating for lack of safety, but didn;t we establish that the amp has a good grounded power cord connected to the chassis?

[trobbins]

Not related to safety matters, but an operational problem may 'pop' up if an earth leakage detection device (RCD etc) is supplying the amp. Sometimes the heater-cathode insulation can exhibit spurious and transient current conduction (I can't recall if the old researchers identified the mechanisms - perhaps from metallic whiskers growing and getting blasted - or thermal expansion induced issues) - which may lead to annoying mains power trips.
Tim