Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gheorge77 on January 10, 2020, 07:46:50 am
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So I'm ready to begin building a Matamp GT120. I've taken the attached schematic and used DIYLC to make a board and layout the amp.
The only changes I made to the schematic were to remove the 4 way drive switch and convert that to a volume know, modeled after that knob in the Weber 6O100 amp (which I did by removing R33, R34 and C27, and moved C28 to the volume pot). I also turned the original volume knob into a ppimv. I used the frondelli MV for that. I also removed the slave and effects loop from the amp, which in the attached schematic means I removed R11 and R12. I'd be happy to share pics along the way if anyone is interested.
The question I had was whether I could put the Depth/FAC controls on a 3 way switch to move the placement in the circuit. The Matamp has the FAC right after the first gain stage. The Orange GRO100 has it right off the input jacks, while the Orange OR120 has it after the tone controls and the 2nd gain stage. The suggested voltage readings on the plates that I have for the Matamp are 140 on V1A, 200 on V1B, and 117 on V2A. How would I go about implementing such a switch?
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So just to update this, I finished the build and on low volume everything sounds ok. Looking forward to cranking it tomorrow. My voltages are higher than the numbers I got from the kit builder but comparable to what 70s Oranges had (~500 on the plates).
The only question I have is this: this uses a 2 pole 6 way switch for the FAC/Depth controls. I used an Alpha switch for it (https://reverb.com/item/6851596-alpha-6-position-2-pole-rotary-switch-make-before-break) and each turn creates a popping sound. I know I won't be turning this switch much when I play. I've also been chasing down a microphonic chassis issue around V1 and the input jack, which is right next to and below this switch. The kit builder suggested using this switch instead (https://www.mouser.com/ProductDetail/Lorlin/CK1590?qs=VJkHNLJxjTdWABv%2FWjynOQ%3D%3D), which is plastic.
I tried adding a 100k resistor to the output of the Alpha switch to try to tame the popping sound, which helped somewhat. Is it possible that the popping is occurring because I am using a non-isolated switch or am I completely off base?
-Ian
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generic answer;
any time you break then make an amplified signal you're in danger of Pop. The cap "charges" when path is open, then "discharges" to "match" the closed circuit. many simple and complex fixes are there for trying just surf up "switch popping" or some such phrase and read n try n read and.....
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I figured that I'd try a pull-down resistor but for whatever reason I only used a 100k. I'll have to try the 1M (which is what I've used many times in pedals, and have gone upwards of 2M2 as well). That shouldn't affect the tone or use of that switch, correct? Also, my bigger question was whether the switch itself could be causing that area of the chassis to microphonic when tapped? I'm very curious to see how it will react when I play it louder and whether that will become a large issue.
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There are several ways to do this. Here is one. The end resistor may not be needed but it is easy and cheap.
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There are several ways to do this. Here is one. The end resistor may not be needed but it is easy and cheap.
Could I get away with 1M resistors at each spot instead?
The bigger question I had was whether it was possible that using a non-isolated switch (Alpha uses metal washers v. the Lorkin being a plastic switch) is causing the popping/microphony?
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Low resistors changes the bass-cut. 10Meg will be negligible. 1Meg.... well that way I happened to draw it may be very little difference. The switch contacts are always isolated on plastic (ceramic, phenolic) so that does not matter.
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I've got some 10M 1/4W resistors... Do you think those would work there or do I need to get at least some 1/2W there?
Could the microphony be coming from the input Jack's? I've got two cliff Jack's that I had to rewire. I was contemplating swapping those for some switchcraft 12a Jack's instead.
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The many-Meg resistors can be as small as your fingers can manage. The power is too small for me to figure.
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So I added the 10M resistors (I actually installed an entirely rewired FAC control with axial caps, which were way easier to wire) and the sound definitely is more or less gone. I had to replace the standby switch because I mentioned to demolish it at band practice last week with a rolling cabinet before I got a chance to play this head above 1.
The microphonic issue still persists a bit, as does the issue of the direction I place the 68k/68k-shielded input ground connection. I'm wondering if it is an issue with the input jacks. I've got 2 cliff jacks on there now...I was wondering if there was any advantage of using new Cliff Jacks v. Switchcraft 12As. I can't find any discussions on the merits of one v. the other.
Of course the other option (which I'd like to avoid) is having shielded wire go from the 2 input jacks to the 68k resistors on the grid of V1A. Any thoughts?
-Ian