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First Tube Amp Project
First Tube Amp Project
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Old 31st March 2019, 07:32 AM   #1
Jostenso22 is offline Jostenso22  Canada
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Default First Tube Amp Project

Hello everyone out there in diyaudio land! I have come across an old Stromberg Carlson Radio (Model 1502) The bad: It was completely filthy, the tubes were all fried and the output transformer was bad. The good: Power Transformer appears to work and it looks quite nice, the speaker is still in good shape albeit 5.5" and 4Ohms the tube sockets appear to be usable, and the chassis looks like a solid foundation. What I am interested in doing is converting this bread box sized Radio into a slightly modified Fender 5F1 champ combo/head. After a few drafts I have come up with a schematic that I think will work. Would anyone here be able to provide me some feedback. I don't know anyone locally that can check my work.
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Old 31st March 2019, 11:10 AM   #2
hex69 is offline hex69  Canada
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Looking at the original 5F1 schematic, here's what I can see.

- The artificial center tap is incorrect, both 100 ohm resistors should be grounded while keeping the tube pin ungrounded.

Click the image to open in full size.

- The negative feedback shouldn't be on the grounded side.

However, please wait to get more advice since I'm no expert.
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Old 31st March 2019, 12:37 PM   #3
Printer2 is offline Printer2  Canada
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Add a capacitor to the first cathode. Tou might want to look at these guys if you are in the US.


Antique Electronic Supply
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Old 31st March 2019, 04:20 PM   #4
Jostenso22 is offline Jostenso22  Canada
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Thanks for the info guys. So it should look more like this? My brain has gone a bit mushy over the past few days trying to hash out all of the details.
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Old 31st March 2019, 04:53 PM   #5
PRR is offline PRR  United States
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The radio has one side of the heater circuit grounded for simplicity and RF stability. The low audio gain and teeny speaker make the hum acceptable. Early guitar amps were this way.

Pretty soon g-amps switched to wiring both sides of the heaters (not using chassis return), which leads to the plan hex69 has posted (which your drawing attempts but not-right).
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Old 31st March 2019, 05:03 PM   #6
VictoriaGuy is offline VictoriaGuy  Canada
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First Tube Amp Project
Quote:
Originally Posted by Jostenso22 View Post
Thanks for the info guys. So it should look more like this?
Quote:
Originally Posted by PRR View Post
The radio has one side of the heater circuit grounded for simplicity and RF stability. The low audio gain and teeny speaker make the hum acceptable. Early guitar amps were this way.

Pretty soon g-amps switched to wiring both sides of the heaters (not using chassis return), which leads to the plan hex69 has posted (which your drawing attempts but not-right).
If you are planning to re-use the radio chassis and tube sockets, my advice would be to leave the heater wiring 'as-is', since one heater tab on each tube socket is probably soldered to the chassis.
It is a PITA to 'un-ground' those tabs, and a lot of hobbyists don't have a big mass 'old school' soldering iron to do the job.
So just delete those 100 ohm resistors on the heater circuit schematic IMO.

If you plan to use the transformers in a new build, on a new chassis, you can arrange the heaters differently.

Quote:
Originally Posted by Jostenso22 View Post
Thanks for the info guys. So it should look more like this? My brain has gone a bit mushy over the past few days trying to hash out all of the details.
The fuse and switch should both be in the 'hot' side of the input AC wiring IMO.

Quote:
Originally Posted by Jostenso22 View Post
Would anyone here be able to provide me some feedback. I don't know anyone locally that can check my work.
If you add your location to your Profile here, it will show under your user name.
This is useful when people are advising on AC wiring, voltages, etc....which differ from country to country....

Quote:
Originally Posted by Jostenso22 View Post
I have come across an old Stromberg Carlson Radio (Model 1502) The bad: It was completely filthy, the tubes were all fried and the output transformer was bad. The good: Power Transformer appears to work and it looks quite nice, the speaker is still in good shape albeit 5.5" and 4Ohms the tube sockets appear to be usable, and the chassis looks like a solid foundation.
If you were able to test the tubes and transformers, you must already have quite a lot of skill and experience.

What voltage (AC) does the power transformer put out on the HV winding?

You chose the preamp tubes because you have 7-pin sockets available?

BTW, you had bad luck if all the tubes and the output transformer were fried- yesterday I got (free) a 1963 Grundig tube stereo console which works and had all original tubes except for one replacement. Tube don't 'wear out' very fast in most cases!
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Old 31st March 2019, 05:44 PM   #7
Jostenso22 is offline Jostenso22  Canada
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The chassis has three mini 7 pin tube sockets along with two octal sockets. I'll have to check my notes for the HV winding if my memory serves me correctly I was getting about 370VAC from centre tap to each side. It seemed a bit high but I took the readings with "no load" if I'm using the term correctly. BTW I am in Nanaimo. Not very far from Victoria indeed.
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Old 31st March 2019, 07:05 PM   #8
VictoriaGuy is offline VictoriaGuy  Canada
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First Tube Amp Project
Adding a tone control moves you to a tweed Princeton circuit, so you can find lots of those online. Here's a variant: GAA's Tweed Princeton Clone

I wouldn't bother with the two inputs that a lot of those early amps have.

You could wire up the power supply and put a resistor across the B+ output to draw 30mA or so to give you an idea of the voltages you can expect. Do you have a copy of PSUD from Duncan Amps? It's useful.

BTW, it wounds like you will be looking for tubes. John Albion at Pacific TV (Victoria) is trustworthy. I've also gotten good service from tubestore in Hamilton, ON. For other components, Digikey and Mouser both have excellent quick and reasonable shipping to Canada. I've bought transformers and chokes from Allied and MusicalPowerSupplies, but shipping iron is expensive. Hammond iron via Digikey/Mouser might be cheaper in the long run.. Avoid Newark- they split my $50 resistor order into three parts and charged $30 shipping for each part. Live and learn.
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Old 31st March 2019, 08:27 PM   #9
Jostenso22 is offline Jostenso22  Canada
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Here are a few shots of the underside of the chassis. Before and after some "tidying up"
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File Type: jpg IMG_0126.jpg (998.5 KB, 224 views)
File Type: jpg IMG_0134.jpg (1.02 MB, 205 views)
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Old 31st March 2019, 11:42 PM   #10
Jostenso22 is offline Jostenso22  Canada
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The rectifier tube glass is no longer vacuum sealed. Other tubes have very toasty looking glass bottles almost as if they were on the verge of melting down. By the way the power supply hi voltage read 378VAC from each side to tap. Thanks for all the advice. I may test out the PSU further with a resistor across the B+ to get a better idea of voltages. I’ll have a look at the PSUD from Duncan amps. I appreciate any good material to help me understand things better.
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