My 50 watt #183 with footswitchable FET build.

[Matt J]

So here's something I've been slowly piecing together parts for and have finally started soldering together: A 50-watt #183 with a footswitchable FET.

Info:
- Chassis, boards, and 1x12 combo by Amplified Nation
- Custom Heyboer #HTS-10359 Power Transformer
- Heyboer HTS-7256 OT and NSC125C1A choke
- Resistors are a mix of Xicon and NTE metal films with some carbon films thrown in the OD section. The reason I went with the metal films was to help keep the noise level as low as possible.
- Orange Drop 6PS resistors.
- 5v NTE relays

I've been working from several adapted layouts, especially when wiring up the relay for the FET.

I've already wired up and tested the PT to make the sure the voltages were ok on the secondaries, before and after the rectifier. I also tested the footswitch this way. I've removed it for the time being to make it easier to work on other parts of the amp.

I'll be posting up several more pictures as I progress alone. If you spot any blaring issues or potential oscillation sources with my wiring please feel free to point them out!!

- Matt J.

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[Matt J]

I used relay sockets this time in case something ever goes awry one of the relays. It's a tight fit in some places, but this way it would be easier to replace a faulty relay instead of having to pull the whole board and try to desolder all of the small contact points.

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[Matt J]

I removed the 1k dropping resistors and placed them inside the footpedal for each LED. Due to the relay voltage being 5v instead of the more traditional 12v the dropping resistor value changed. I used a small pot to help dial in the correct amount. They ended up being about 330r for each LED.

Up next, biasing the FET, wiring it up to the relay, and finishing the front panel pots!

- Matt J.

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[Matt J]

Well.... I just attempted to set the FET bias and things didn't go so well. Not long after hooking up the two 9v batteries the capacitor popped right in front of me on my bench. (Thank God for always wearing safety goggles)

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So, obviously I screwed up something major (and scared the bejeezus out of myself and my cat that sitting in the floor near me)

I think I'm just going to leave things "as is" and keep the stock values listed on most FET layouts. I'm using a NTE 452 FET and really hope what just happened didn't kill it!

- Matt J.

[M Fowler]

Clean looking built so far, nice.

Mark

[UltraHookedOnPhonix]

Nice build Matt, keep it up!

Sorry to hear about the FET circuit. I would put in a new e-lytic and hook it up to a breadboard for testing. Double-check all polarities. From the 150K/8K2 divider, expect to see around 16-17V.

In my 183 I got the following voltages with a 4.32K Source resistor:

B+5: 313V
150K/8K2 divider: 16.22V
NTE452 Drain: 10.58V
NTE452 Source: 2.47V

Remember that the volume will drop slightly with voltages at the Drain above 11V.

Also, the lower the FET's Source resistance, the lower the Drain Voltage and vice-versa.

Good luck!

[Matt J]

Nice build Matt, keep it up!

Sorry to hear about the FET circuit. I would put in a new e-lytic and hook it up to a breadboard for testing. Double-check all polarities. From the 150K/8K2 divider, expect to see around 16-17V.

In my 183 I got the following voltages with a 4.32K Source resistor:

B+5: 313V
150K/8K2 divider: 16.22V
NTE452 Drain: 10.58V
NTE452 Source: 2.47V

Remember that the volume will drop slightly with voltages at the Drain above 11V.

Also, the lower the FET's Source resistance, the lower the Drain Voltage and vice-versa.

Good luck!

Thanks a lot for the input, Jonas! Your #183 build certainly helped inspire mine!

I've been looking over what might have gone wrong - the electrolytic was installed correctly, but I think I may have connected the batteries up incorrectly with the positive charge feeding into the negative and vice-versa. I don't know if that could have caused the capacitor to overload, but something sure set it off!

Of all of the information on Dumble amps, the FET circuit is the thing I probably understand the least. I've usually omitted it from previous builds. This time, though, I wanted to challenge myself and work towards getting one that's operational in this amp with a better understanding of how the circuit operates.

To play it safe, I've just ordered from NTE a new 452 transistor, as well as some replacement capacitors and resistors. I'll desolder the board, clean it up, and start over from there.

In the meantime, I've nearly finished wiring up the rest of the front controls. I'll start looking at the power supply board after that.

- Matt J.

[dorrisant]

Very nice build Matt!! Looks like we use the exact same parts...

Jonas, what do I need to understand this FET circuit. I have use Martin's document with the batteries and loved the result. Your numbers make me want to pull my chassis back out and tinker. I'd love some advise on what the targets are and why.

[UltraHookedOnPhonix]

Very nice build Matt!! Looks like we use the exact same parts...

Jonas, what do I need to understand this FET circuit. I have use Martin's document with the batteries and loved the result. Your numbers make me want to pull my chassis back out and tinker. I'd love some advise on what the targets are and why.

Hey Tony! The FET ckt is your basic common source amplifier. The 3M3 resistor holds the gate at ground potential. The 1M/100pF forms a HPF with a cutoff frequency of 1.6kHz. The source resistor develops a voltage gain across the 3M3 resistor to keep the source above ground. Values of approximately 1.74K and below will create cutoff (lower than 2.45V at the drain). The 4u7 source bypass cap increases gain at AC and the cutoff frequency is determined together with the source resistor, which is well below 20Hz in typical situations. The 10K drain resistor develops the output voltage across it, and the 4u7 output cap couples the AC to the next stage, loaded by a 10K volume pot.

It’s kind of hard to know what HAD was shooting for in terms of a bias voltage, as I’ve seen him use source resistance values of 2K7 (BBC-1) and 8K2 (#124). The value of the source resistor is often obscured in pics.

Common practice around here has been to adjust the source resistor for around 10-11V on the drain. Hopefully, someone on here can really give you a breakdown as to the "why."

Smallbearelec has a great article on JFET biasing here: http://diy.smallbearelec.com/HowTos/Bre ... areAss.htm.

[Matt J]

Thank you for your very kind and informative responses, Tony and Jonas!! It's a lot to read over and digest about the FET circuit. Looks like I have some more research to do!

Made a little more progress last night after work. Finished up the front control pots and the shielded wiring. Flying leads to the OD section were measured and trimmed. Gave everything a run through with the continuity function on my DMM to make sure none of the shielded wire leads ground out.

Per recommendation from Taylor at Amplified Nation, most of the soldering in this amp is with Kester 245 solder. It flows a lot cleaner and seems to create a better connection, in my opinion. It doesn't adhere to the back of the pots as well as 66/44 solder, so I had to use that to ground the shields. You have to spend a while scraping and cleaning away the rosin that builds up afterwards though to clean things up a bit.

- Matt J.

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[Matt J]

Thinking ahead, one thing that has popped up in my mind is wiring up the output transformer.

While I've never experienced it personally, I would like to avoid wiring up the OT primaries backwards and having to deal with the squealing that can come afterwards. Does anyone know of a decent test you could perform beforehand to help see which OT lead is the correct one for V4 and V5, or is it really just a crap shoot?

I'm using a MOP50 by Heyboer I purchased from Hoffman. Wiring diagram is attached.

- Matt J.

BRITISH-Plexi-800-50W-OT.pdf

[sluckey]

I always just temp solder the full length plate leads to the sockets when dealing with an amp with NFB and an unfamiliar OT. Even if the amp sounds OK I always swap plate leads to be absolutely sure which way is correct. Getting the phase wrong does not always result in a squeal or howl. Sometimes the effect is much more subtle. Depends on the particular circuit. Only after I'm confident which way is correct will I trim leads to correct length and make a permanent solder connection.

[Matt J]

That's probably what I'm going to end up doing, Sluckey. I'll give everything a listen, swap things around, do some A and B comparisons, then make the final connections and tidy things up from there.

Special shout out to Taylor at Amplified Nation for the chassis and 1x12 combo. I did some more test fitting this morning now that the faceplate is all wired up. It looks fantastic! A Scumback 1x12 will eventually go into this. I wanted to go for a very early ODS style look and I'm super pleased with how this came out!

Back to working on the FET this afternoon after work.

- Matt J.

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[norburybrook]

lovely. If it's made from 18mm (that's 3/4 inch to you lot!) plywood it will be heavy regardless of speakers.

I've just put an alnico in mine having taken the EV out but it's still heavy.


Your build looks great Matt, a lot neater than mine



M

[stelligan]

Nice looking amp! Retro cool.....