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Almost a year ago I built my first amp, an 18 watt TMB based off of the BBQ layout/schematic. I sourced all my own parts and used Hammond transformers (270EX power & 125E output). It has a switch for the output impedance so I can run it at 5 or 8k at 4, 8, 16 ohms.

The amp is super loud, especially if I run it through a 4x12, so loud in fact that I never crank it up full. The amp is pretty bassy and dark sounding on both channels (controls do not seem to off as much tonal variation as I would like), but the TMB is slightly brighter sounding. Problem is, the TMB channel gets really loud early on the volume control, and is unplayable around 3 - 4 because it starts feeding back really bad.

Wondering if you guys can offer me some suggestions on solving:
1) the feeding back problem
2) the high volume problem (or do the Hammond transformers up the voltage a bit and make it louder???)
3) getting more tonal variation

Would it help to post some pics?

Thanks in advance guys,
Phil

I just finished my sIII with a Hammond 270EX PT. The voltages were very close to those measured with the Trinity PT. I don't think that would be the source of the problem.

For problem #1 - does the feedback occur with your OT switch in both 5K and 8K positions? A common cure for this type of problem is to switch the OT connections (the blue and brown wires) to the power tubes, or in your case the 5K/8K switch. That's complex switching - you might try bypassing that switch for now with a straight connection of the blue and brown wires to the power tubes to simplify troubleshooting.

For problem #2 and #3 - This is a loud amp - I'm not sure what you're used to volume-wise. However, you may have bypassed your tone controls somehow. This would make the amp louder because the signal is not being attenuated by the tone stack (like a 'raw' control). Check all of your grounds in the tone stack area. Make sure the wires are connected and re-flow the solder joints to the mid pot and volume pot on the TMB side and the tone pot and volume pot on the Normal side. Re-verify your entire grounding scheme as well.

Pictures are always a big help.

+1 on the reverse lead from OT. I'll post a Tone Tweaking Guide you can look over. Not sure what the BBQ schematic is, but the tone tweaking will help you out there. No comments from the BBQ group?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Well I'm used to loud amps - I have a couple vintage Super Reverbs and a '66 Bassman that I love to crank up nice and loud!!!

If you can post a tone tweaking guide that would be great! I did notice the BBQ layout is slightly different altho I'm not sure exactly what those differences may do to the overall tone of the amp (the schematic and layout is available from the BBQ website, not sure if I should post a link here???)

The feedback problem occurs with the impedance selector in all positions, BUT its only on the TMB channel. I can crank the normal channel to 10 without any feedback - I would think that means the OT leads are fine?

It probably won't be the prettiest of all the builds on the site, but please keep in mind I'm a newbie and this was my first ever try at an amp build (or any other electronics for that matter!!!) Here are a couple pics of my amp:

If you getting squeal on either channel. try swapping the OT leads. Also, twist them from OT to tubes.

I thought I posted a tweakers guide here on the forum somewhere..

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I've built about a dozen 18 and 36 W TMBs using the BBQ layout. I've never used a 5K or 8K OT though. The kit calls for a 4.2k OT in both cases and that has always worked well for me. I wonder if your tone circuit is wired correctly?

Have you tried wiring the OT direct to the EL84s one way or another without using the switch? Keep things simple and track down where the trouble is coming from.

It looks like the output tube wires are all run together in a bunch. Has that worked for you guys? I generally try to keep those separated, especially when running 4-EL84s, it can bring in a bunch of distortion and squealing etc. The plate wires particularly keeping them separate from cathode and grid wiring. Everyone comes up with their own methods I guess, but this has always worked well for me.

One more thing, the wire from the volume control center lug to the phase inverter, I always run that using ground shielded audio cable, it makes life easier. I always ground shield the input jack wires as well in all amps. Good luck, hope this helps.

That's great. It helps me to solve my problem. I want to ask some questions about it. Could you please contact me on GBWhatsapp? I shall be very thankful for this.

Can you find your posted a tweakers guide again, @coco? I want to see it.

Feedback Problem: Excessive gain in the TMB channel could be a factor. You might try a lower-gain DragonSweeper preamp tube (like a 5751 or 12AY7 instead of a 12AX7) to tame it a bit. Also, check your lead dress—moving high-gain signal wires away from high-voltage wires can help reduce unwanted noise.

High Volume Issue: Your transformers may indeed be running slightly higher voltages, increasing headroom and volume. If you want smoother volume control, consider using a different taper for your volume pot (an audio/log taper pot might help). A master volume mod or a voltage-dropping resistor could also be options.