100W HRM in the BluesMaster Style?

[mdroberts1243]

I've finally completed my BM build in Funk's chassis... except for mixing up the input & output on the FET board and a dirty shorting switch on a switchcraft jack it came right up.

I've posted a couple of pictures... this build is a bit unique/experimental as I've installed some silver-plated vector spring clips to allow for swapping out all the preamp plate and cathode components (since there doesn't seem to be one definitive set). I also have a switch to short out the controversial pre-HRM coupling capacitor and I have a variable power resistor for the 'robben ford' mode selected by the DPDT standby switch.

Just trying to dial in the voltages right now by making adjustments to the dropping string and then I want to plan a series of experiments on the preamp components.

[mdroberts1243]

Hi,
I've finally settled on a set of dropping resistors that I think is a reasonable compromise for my wild swings in line voltage... from 115 to 122 depending on the season and the whim of Ottawa Hydro.

I will post an updated schematic in the next couple of days, but here are some changes I've made during the bring-up of the amp...

Bias voltage: I wanted to be able to swing from upper 50s down to lower 30s in voltage. I've changed the 3.9k to 1.5k and the 27k 'tail' on the pot to 15k. The 30s range was so I could try out 6V6s if I want.

FET Bias: Changed the 5.1k to 12k to get just above 10V on the Drain of my NTE452... your results might vary.

I've stuck with my 3.4K 1st dropping resistor, which gives me lower B+3 then others but I think the PI is in a good operating range.

PAB... Too much boost! I decided that 6dB of boost at the speaker in clean mode would be something to aim for... the 22M that normally is installed to reduce pops was replaced by 72k (Mid-pot tail to ground) and the resulting tone-stack 'lift' provided 6dB boost in PAB mode. Others have noted 470k to remove some harshness from the boost or 82k (about 8.5dB boost).

Preamp dropping resistor... the last dropping resistor (10k from B+4 to B+5) has been replaced by 2.2K to bring the V1 and V2 voltages closer together and stay in the ~200V sweet spot on the plates.

I went with the Fender 500 ohm screen resistors instead of 1000 ohms, giving up on the idea of trying EL-34s.

The 1st set of filter capacitors is now prior to the standby switch.

I missed two pieces of coax on the schematic... between OD1 and the OD Drive pot and between OD Volume pot wiper and the OD/Clean relay.

I modified a marshall 3-button footswitch to work with the design and used NTE 5/6V relays to ensure I had margin for the LEDs to work in series with the relays.

Here is a list of my voltages as the are now (for 116.6VAC and 120.8VAC):

Code: Select all

Line:   116.6      120.8
B+1     409        422
B+2     408        420
B+3     381        391
B+4     306        314
B+5     298        306
V1a     183/1.66   188/1.72
V1b     190/1.56   196/1.61
V2a     198/1.59   203/1.65
V2b     204/1.47   210/1.51
V3hi    237        244
V3lo    231        235
V3k      39         40.4
Vfet     10.37      10.71

[heisthl]

Very nice looking build - keep up the good work!

[ayan]

Mark, what is the P/N number for your trimmers? (any value will do so that I can locate the part). Thanks!

The amp looks great and what a terrific idea to make it "parametric" like that!

Gil

[JimiB]

yes beautiful thumbwheel trimmers. I looked for 45 min the other day and nothin!

[mdroberts1243]

Mark, what is the P/N number for your trimmers? (any value will do so that I can locate the part). Thanks!

The amp looks great and what a terrific idea to make it "parametric" like that!

Gil

I'm not the best one to ask about trimmer pots! The ones I tried to order didn't fit and I got mine from local surplus outlets, including the old grungy carbon open frame ones.

The HRM 1M is a Piher trimmer marked "1M 136"... I think these are 9mm laydown top adjust, but don't trust me!

The vertical mount are CTS Thumbwheel pots... the 100k is marked "R1379105 100K" and the 10k ones are marked "X201R103B DJF1".

The silver plated spring clips were a last-minute decision when I stumbled across a bunch of them in my parts drawers... so far they've worked very nicely, they cleaned up nice with silver dip and they are making really good contact to the component leads... no noise or intermittent connections. But, longer term I'll have to settle on one set of values and re-work the board.

[mdroberts1243]

Very nice looking build - keep up the good work!

Thanks Heisthl!

I really appreciate the detailed photos you and Funk have posted... I referred to them constantly trying to work out the layout and routing for this project.

[ayan]

The vertical mount are CTS Thumbwheel pots... the 100k is marked "R1379105 100K" and the 10k ones are marked "X201R103B DJF1".

Those are the ones I meant, they look great and very legit. Not surer where I can buy them, but I will keep them in mind for the future.

Gil

[mdroberts1243]

EDIT: Look for a more recent schematic posted.

Attached is version 10 of the schematic, this reflects what I've actually implemented so far. I'll also post at the head of this thread.

Note that I used NTE R40-11D2-5/6 relays to guarantee sufficient margin of operation for both 12V manual switching of the relays and series LED footswitch activation.

I want to add a PAB lift to the Jazz mode circuit... I'll try out what Gil suggested and also just a straight lift of the 'jazz' cap from ground.

I want to try some of the options for plate/cathode/cap values with my 'spring-loaded' main-board... but I need a plan!

[mdroberts1243]

Here is Revision 11 of the schematic (version 0.99).

I've gone with the spreadsheet 'sunnyland' values which may be or not be the values used in Sonny Landreth's Dumble.

I found in experimenting with the spring clips that the Rk values seemed to have been crossed on each tube and the 'correct' ratio values performed better... so that is what is in the schematic.

To make a general summary, the plate/cathode resistor values in the bluesmaster (as captured here) are 'cleaner' than a 'stock' HRM... the 1st stage enters compression later and has much more clean range overall. On subsequent stages the associated controls have a wider range of 'compression', except for OD1.

[ampdork]

Dr. Roberts,

Any idea where one might find those spring clips? Much better than my own method of wiring in switches and pots and junk. Great work. VERY cool!!!

[mdroberts1243]

Dr. Roberts,

Any idea where one might find those spring clips? Much better than my own method of wiring in switches and pots and junk. Great work. VERY cool!!!

I'd think they'd be pretty rare now... they were a vector prototyping product many many years ago... I stumbled across a few in my parts cabinet and cleaned them up with silver dip.

I enjoyed using them on this build, but they were a bit of a PITA to remove and rework the boards... I'm thinking of making up some prototype tube socket boards with the ones I have so I can do more bench work.

[ic-racer]

Thanks for posting your amp and schematic. I have a question about your FET input jack. I searched the Switchcraft site and it looks like it is type XVII on their schematic sheet. But I must be wrong as the XVII does not come as an enclosed jack like you have. Can I ask you where you got your input jack?

Thanks

[greiswig]

Ingenious building, Mark! Some very good ideas for easy tweaking in there. I'm hoping someone more knowledgeable than I will chime in and talk about the cathode and plate leads on V2...they're supposed to be run a certain way to interact with one another some, but I've never seen a definitive "this is how to do it" shot.

[mdroberts1243]

Thanks for posting your amp and schematic. I have a question about your FET input jack. I searched the Switchcraft site and it looks like it is type XVII on their schematic sheet. But I must be wrong as the XVII does not come as an enclosed jack like you have. Can I ask you where you got your input jack?

Thanks

I ended up getting an enclosed stereo jack (no-name brand) with isolated switches and wiring those to get the same functionality... in hindsight, I think you can use a simple shorting jack like I've seen in other layouts.