#183 w/Switchable HRM, Almost Finished

[David Root]

This is the most complex amp I have yet built. 6-way FSW is the only thing I have left to wire up. Besides that and the HRM board, it differs from the original 183 quite a bit:

Lots of Piher resistors.

NOR/FET selection footswitched, with face mounted gain pot.

Switchable HRM/Non-HRM--thanx to Shad, that saved me a LOT of work!

Cat5e footswitch cable--thanx to Gary of Glaswerks!

Separate Clean & OD MVs.

Inboard buffered loop.

Custom Heyboer PT w/separate 6V3-0 winding for 5V relay power.

M-C 40-18026 100W Marshall JMP OT

[Structo]

Yep, that is pretty busy all right.
It is amazing the amount of wire and soldering in these amps.

What is the little board with the six resistors on it?

[David Root]

Those are the 1K current limiters in the relay PS circuit. I didn't want them all flying off the relay PS board +ve out.

[M Fowler]

Nice job David, six way foot switching wow.

Mark

[alvarezh]

Very well done David!

I am going to guess on the features of your next build…. a pair of WiFi transceivers, one for the footswitch and another for the guitar!

[David Root]

Heh heh, never thought of that!

[CHIP]

Great job David.

[boldaslove6789]

Nice David ! Killer build, should be hella versatile.

[Bob S]

Wow - I am not worthy to comment, but very impressive.
Well done.

[rogb]

Love the build David! A lot of work there, well done.

6 way switching - now that's my kind of build, hope it sounds awesome too!

[bepone]

very nice.. one question, how FX loop interacts with other circuits inside (do you have "smoothing" effects also when fx loop is built inside?)

[Colossal]

Sweet David. Very nice looking build!

[ChrisM]

Where are all the relays?!!!

What's the footswitch look like?

[David Root]

Thanx to all of you, it was indeed a lot of work. I just hope I didn't leave any ground wires unsoldered, etc. as some of the ground points became overrun with adjacent wiring, especially #1 ground.

The relay power wiring was time consuming as I left it a bit late, should have laid it out much earlier, and so had to wire around some heavily connected relay boards.

bepone, not sure I follow you on "smoothing effects" with the buffered loop. Are you referring to possible capacitative high end losses from interaction with adjacent components? If so, I'll soon find out, but I have used the very same loop twice before in a 2nd gen build and a Bluesmaster w/HRM without any audible problems, at least to me.

My high end hearing is not as good as it used to be, however this time I manually switched it so I can check it out of circuit vs. in series circuit. I use Normster's abbreviated version which is in his '97 ODS layout in the Dumble files. It is set up I think primarily for a 12AX7 but I like to use a 12AT7, lower impedance and a bit more current seems to be good in a loop buffer tube. (I got that suggestion from KOC in TUT 1). It works well and takes up a minimum of chassis & board space. The two PS caps for it are on the upper left of the main PS board, and run off the screen node.

Chris, the relays are: OD and PAB are the small orange ones on Jason's HRM board. The other four are in line between the main board and the pots. Larger black Omrons, from right to left, NOR/FET, MB, Brite and HRM/Non-HRM. They are all 5V. Thanx to Colossal for those four boards, they are his new production and just fit nicely! Their PS is on Jason's Relay/Bias PS board on the left chassis wall.

Next time I'll look at JFET switching, six electromechanical relays is a bit much.

The footswitch is two three-button AES units bolted together end to end and rewired to have the LEDs on when relay is on. It's rather bulky but there are no 6 way aftermarket units available that I could find.

There is no provision in the HRM/Non-HRM switching for impedance adjustments etc., it is just the straight circuit that Shad posted in a recent thread, so I'm not sure how well it will work. We shall see.

[bepone]

bepone, not sure I follow you on "smoothing effects" with the buffered loop. Are you referring to possible capacitative high end losses from interaction with adjacent components? If so, I'll soon find out, but I have used the very same loop twice before in a 2nd gen build and a Bluesmaster w/HRM without any audible problems, at least to me.

My high end hearing is not as good as it used to be, however this time I manually switched it so I can check it out of circuit vs. in series circuit. I use Normster's abbreviated version which is in his '97 ODS layout in the Dumble files. It is set up I think primarily for a 12AX7 but I like to use a 12AT7, lower impedance and a bit more current seems to be good in a loop buffer tube. .

hello david, i think that dumble used coax cables to fx loop, inside and outside of the amp , (AMP->Dubleator->AMP) also to smooth things (if is OD too harsh).

if you build fx loop inside the amp, you don't have coax capacitance "on your side".. so i was thinking that in this case smoothing effects are reduced because of too short connecting wires. i don't know is it true, i'm very curious, please let me know.

fx loop with 12AT7 is better option for driving effects, maybe more transparent? be shure that you change anode resistor to 47K, and cathode 470R for this option (first half 12AT7), to have good working point and gain advantage of smaller output loop impedance .

12AX7 in effects loop (from my experience) works for the base tone of the amp, and 12AT7 not that much..

it will be interesting too hear your feedback, regards