An Oldie but a Goodie...

[tubedogsmith]

The late '90's style 100 watt HRM. These were the first real Dumble amps I got to spend quality time with and it was 1999. I spent time with three of them and they were all essentially the same. The were cool amps but really didn't knock me out. The amps I built from them came out likewise.

I kinda just forgot about Dumbles for a while. Fast forward some years and I got to spend some time with an '80's amp that I didn't get to open up but judging from the sound and control layout had to be a non HRM, Skyline amp. So easy to play and it was the first time I got to hear what the dumbleator would do for one of these things. Just fell in love with this sound and wanted to learn all I could about this type amp. After trying all the various variations I still like the "basic" highplate skyline non HRM the best of all this non HRM style amp.

A few weeks ago a friend traded a Bluesmaster style amp I'd built him back in towards something else. When I cranked it up it really sounded great, that fat lead tone that this type amp does. But a few days passed and I decided to convert it to my now absolute favorite circuit, the HRM skyline. Bluesmaster is a cool amp for sure but not subtle at all really, at least compared to some of the others.

I did this conversion in stages a few days apart so I could really hear and verify what all these tweaks did. First stage I changed the slope resistor to 150k but left the plate resistors on the first tube at 150k and 120k and cathode resistors at 2.2k and 1.8k. Changed the bass cap to .1 and mid cap to .01. I left the gain circuit with the 100k pot, 120k resistor, 470k resistor bypassed by a 47pf cap. On the OD tone stack I changed the slope resistor to 100k and the treble cap to 330pf. Reset the treble trim to 92k, bass at 135k, and mid at 5k. I then swapped out pots putting in the standard skyline values. I left the presence circuit and phase inverter and played it that way a few days. It was bright and kinda boomy. some good qualities but not really great. I ended up back the OD treble down to 70k and bass to 95k. That helped with some of the problems but made it congested sounding. Tweaking controls helped but I didn't really dig it.

Started the next session with changing the presence pot to 2k and the rest of that circuit to the 390 ohm, 4.7k, and 1uf cap. I change the PI caps to .05. Now I had a huge improvement and really liked it a lot. Much more improvement than I expected. I played it like this for over a week and a couple times almost put it back in the box. But, I still had this niggling deal going on at times that it wasn't the best I'd heard or done.

So, sat down with it again this past Sunday and finished it. I changed the V1 plates to 220k and 150k, and cathode resistors to 3.3k and 2.2k. I changed the V2 cathode cap to 1uf. I put 330pf snubbers on both sides of V2. I changed the PI cathode resistor to 850 ohm and the tail resistor to 24k. I reset the OD tonestack to 92k, 135k and 5.5k. Set the internal gain trimmer to 30k. Played it a few minutes and just broke out in a stupid grin.

I stumbled on this mixture of values and settings about two years ago and it's been amazingly repeatable for me. Sometimes I put a cap, 100pf to 270pf on the hot lug of the OD master to take a little highend off but no always.

Anyway, this circuit has a great clean, great boosted clean, killer OD and the OD with boost is just plain sweet. It has really bested any of the non HRM circuits for my personal preference. I think it does what they do and better with more versatility. Anyway, if any of this is of value to anyone great. This really is an under appreciated circuit I think. Here's a shot,

[dcribbs1412]

Great info tubedogsmith
I'm about to tweak a HRM and this will be some very handy info/tips
to try.
Really appreciate it
Thanks

Darin

[stevlech]

Very kind of you to share your recipe.

In the photo, it looks like you still have the same OD entrance circuit- is that true?

Nice job.

Have a good day

[tubedogsmith]

Very kind of you to share your recipe.

In the photo, it looks like you still have the same OD entrance circuit- is that true?

Nice job.

Have a good day

Yes, it's a 100k trim pot, 120K resistor and 470k risistor with a 47pf bypass cap.

Very smooth amp and really captures the non HRM thing +. And, the circuit works very well with a neck humbucker and strats.

[stevlech]

Thanks for the reply. Sounds like my kind of amp. Someday I will work up the courage to build a D-style amp.

[boldaslove6789]

Very kind of you to share your recipe.

In the photo, it looks like you still have the same OD entrance circuit- is that true?

Nice job.

Have a good day

Yes, it's a 100k trim pot, 120K resistor and 470k risistor with a 47pf bypass cap.

Very smooth amp and really captures the non HRM thing +. And, the circuit works very well with a neck humbucker and strats.

Build looks steller, great job!! Actually glad to see all the HRM's being posted lately, the are possible the hardest circuit to tweak to sound exactly right compared to all the others.

Bet it sounds as good as it looks inside and out.