Building a kit from Mod Electronics

[Brokegunner]

Hi all, I am new here and this is my first post. My name is Brian and I am in NorCal.

My brother is a guitarist so I want to build him a guitar amp for his birthday. Since this is my first attempt at building a guitar amplifier, I figured a kit would be the best way to go.

I was on the Antique Electronics Supply website and bought a kit called the 102+ from MOD Electronics. I have been trying to search this forum for any threads on this kit but the only results are on a "102" which makes me think they are discussing a different amplifier. Is there anyone on here that has experience with the MOD 102 or 102+, or even any suggestions on a discussion forum that focuses on the MOD kits? Here's a link to the description of the kit:

https://www.modelectronics.com/product/mod-102-0

While I have very little audio amp experience, I am very experienced with RF Amplifiers and tube radio repair. My main hobby is rebuilding old ham radio gear and test equipment, so I believe I have everything I could possibly need to get this kit assembled.

Thanks!! Brian aka Brokegunner.

[sluckey]

Looks like a simple straight forward amp circuit with a few extra switches added for flavor. Instructions are well written in the old Heathkit style. I don't foresee any problems building this kit.

[Phil_S]

FWIW, I agree with Steve. There are pictures and whatnot on that website. I assume they included a schematic and layout diagram. This shouldn't be any different from building a ham radio. The same general build rules will apply - lead dress, fixing components, etc. Your stated experience suggests you can do this easily. If you have questions as you get into the build, it's OK to post them here, and do your best to include photos.

[JPK64]

Agree with all of above. Those are really good instructions. You shouldn't have any difficulty. If questions pop up about the theory of the amp etc, I'm sure the gurus here will help.

[statorvane]

I have been trying to search this forum for any threads on this kit but the only results are on a "102"

Probably because no one has had trouble building this amp with the supplied directions. Those are very good instructions and well illustrated. Far more info there than the news in my Sunday paper.

[Brokegunner]

Well I'm back finally to report I got the amp finished and it sounds great. I'll post up a photo of the inside tomorrow but here's the finished product.

20221210_100351.jpg

One thing I'm curious about is the danger of damaging the amp by not having a load connected? I read a forum post where someone changed the output jack to one that shorts the signal to ground when there is no plug in the jack. Since I'm using an external speaker cabinet I'm worried someone may turn this on without the speaker connected but not sure if that's a valid concern on a tube amp?

[Phil_S]

Some people use a switching jack with a resistor to simulate a load when nothing is plugged into the output jack. It is a real concern to operate the amp with no load and this will protect the output transformer. When you plug in the speaker, the switch breaks the connection to the dummy load resistor, allowing the amp to operate as if it is not there. As I understand it, the value of the resistor isn't critical, but don't make it real high. For that amp, I'd guess 2W rating is OK, but certainly no harm if higher rated. Basically, use what you have on hand, within reason. And congratulations on the build!

[Brokegunner]

Some people use a switching jack with a resistor to simulate a load when nothing is plugged into the output jack. It is a real concern to operate the amp with no load and this will protect the output transformer. When you plug in the speaker, the switch breaks the connection to the dummy load resistor, allowing the amp to operate as if it is not there. As I understand it, the value of the resistor isn't critical, but don't make it real high. For that amp, I'd guess 2W rating is OK, but certainly no harm if higher rated. Basically, use what you have on hand, within reason. And congratulations on the build!

Thanks for the reply. I have some 20 ohm 1W resistors so I'm thinking 2 in parallel will give me 10 ohms @2W and I found a shorting jack in my parts drawers.

Here's the jack:

20221211_142541.jpg

Here's the inside of the amp:

20221211_142349.jpg

[Stevem]

You can have a tube amp on forever without a load on it even without a shorting jack, just do not play anything thru it.

[Phil_S]

I have some 20 ohm 1W resistors so I'm thinking 2 in parallel will give me 10 ohms @2W and I found a shorting jack in my parts drawers.

10Ω is good if your OT calls for 8Ω. If it is a 16ΩOT, you might want to use 4 of them in series/parallel to get to 20Ω 2W. It looks like you have plenty of room. You could add a terminal strip if needed to simplify wiring on the jack.

Remember what Stevem says, it's only a problem if you run a signal through it without a load. With nowhere for the signal to go, it turns around and heats up the OT causing it to fail.

[JPK64]

That's a pretty cool kit. Very reasonably priced. Love the point to point layout. My first amp was point to point. AX84 High Octane. I'm staying with turret boards now, much cleaner. Glad you got it working.

[R.G.]

One thing I'm curious about is the danger of damaging the amp by not having a load connected? I read a forum post where someone changed the output jack to one that shorts the signal to ground when there is no plug in the jack. Since I'm using an external speaker cabinet I'm worried someone may turn this on without the speaker connected but not sure if that's a valid concern on a tube amp?

It's a valid concern, but not particularly so for this amplifier. In other amplifiers, it can be a very big deal.

For amplifiers that use negative feedback from the speaker to the output tube(s) driver, the gain and phase shift of the power amp can change significantly with no load. Some amps with this power amp feedback can go into oscillation without a load, and this can sometimes get bad enough to kill the output tubes or even the output transformer. This amp has no feedback in the power amp, so it's not prone to this particular problem.

Enough tube amps have been damaged by running with no load that the idea that no-load is deadly to tube amps has become ingrained in the mystical lore of tube amps. Solid state amps have the reverse problem: they run fine into open circuits, but can be damaged by loads with too low an impedance unless they incorporate self protection circuits.

As a builder of RF equipment, you're uniquely suited to understand the change in gain and phase for no-load conditions causing oscillation.

Congrats on your new amp!