Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shortfuse on November 12, 2010, 09:14:33 am
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I decided to by pass the Stout for now and build the amp I wanted to in the 1st place which is the plexi 50.
I have all my parts chassis punched and the board is populated but nothing is soldered yet as I was going to install the Hot Switch Mod.
But back a few weeks ago I made a comment on Knotscott's Stout post about doing so in a Stout and there were several cautions about doing this. Doug even came on and said wire it full time because of noise. Is this only for the Stout or does this hold true to the plexi as well. I like the idea of having a master volume amp at he flip of a switch and going right back to the standard preamp mode.
The mod if put in an input jack seems easy enough to add or remove if I make previsions for that in the build as I go.
Please give me all comments Pro's Con's / the good the bad so I can weigh my decision I wont take any comments a negative just as you opinion which we are all entitled to. I would really like to hear from some people that have actually built this amp with and without the mod.
Thanks in advance
Steve
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Hi shortfuse,I built a version of the plexi and added the hotswitch and wasn't that wrapped in it.The guys came up with the TOS which the OD was alot smoother and was adjustable. I have not done this mod to the plexi ( or added the PPIMV).Thanks
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I just leave the hotswitch on full time in all the amps I have it installed in
You can go from clean to overdrive by just adjusting the ratio between volume and master.
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Thanks Doug
I made some progress this weekend and got the chassis all punched out. That PT is some pretty tall iron just about makes it in the head cab I built.
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Finished up the amp tonight used the lamp limiter this time thats a good tool. Wish I did that on my 1st build.
Anyway everything seems to be ok but there is no sound? I am sure I did something dumb or easily over looked.
It's late and I will start the process of backtracking & elimination again with fresh eye's tomorrow.
I did fill out the voltage chart. Not sure if this is how it done but http://www.el34world.com/forms/valvedata/SFHP50.htm (http://www.el34world.com/forms/valvedata/SFHP50.htm) link seems to work.
While taking the voltage readings I did notice a very, very faint scratchy squeaky kinda noise coming from the speaker when reading pin 4 of the power tubes.
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Assuming you have everything wired properly from the output tube plates through to the speaker jack, try this:
Does your meter have alligator clips? If so, clip the black lead to ground, and probe with the red lead (carefully and with 1 hand only!) with the meter on its highest voltage range.
You will start at the plate of the output tubes. As you touch the meter lead tip to the tube pin, you will hear a slight pop at the speaker. Next try probing the output tube grid; you will hear a similar pop, maybe slightly louder.
Move back to the phase inverter grid; the pop will be louder still. Keep moving back towards the input, touching the meter lead to tube grids. The pop should get louder each time you move back towards the input. If you find a stage where you don't hear that little pop, you've likely found the area with the problem (between the point of "pop" and "no pop").
This all assumes d.c. voltages appear ballpark-correct, and there are no shorts/opens from the power tube plates through to the speaker jack.
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Humm that said It should have popped while I was taking the voltage readings last night. The voltages are in the link in my previous post but I copied and pasted to the post now. Seeing as though I am still very much a newbie I really don't know if they are in the ball park or not. I seem to recall a diagram somewhere on the site that had the plexi 50 with voltages at certain points but cant seem to find it again.
Thanks for the advice.
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Ok I have done something wrong "again" I checked as suggested and the only thing i hear is a faint squeak from the tubes when pin 3 on either tube or any thing following pin 3 to the board is touched with the meter. I did find that I did not have the heaters connected to the 100 ohm resistors the 2nd pas through but that was it.
I have to take a break I am staring to get blinders on.
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I found it and like I predicted it was something dumb I wired the speaker ohm selector wrong and didn't notice it before because I was sure I had it right. Thank God for the library and some more reading and Butterylicious for harping on me about the lamp limiter. Now to re tidy what I took apart to trouble shoot and jam for a while. Will post pic's later.
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This amp is better than i expected it was a great build very straight forward with all the info I needed on the site. I just ordered the pieces to try the hot switch mod. Cant wait to really let it rip tomorrow had to tone it down a bit tonight. There are things I would do different if I build this one again as far as layout. I really like the KT88's tried it with EL34's & KT77's and like the KT88's the best.
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Well I finally got to play it last night with the volume up and all I can say is this amp exceeded my expectations. I can't wait to get it home and place it on a full stack. I am not so sure I want to change a thing and love the sound of being able to blend the 2 channels jumped in the front. This amp sounds great with the Tone Bone, Big Muff, or BB+ in front of it. My parts for the hot switch mod will be here next week. I think I will wire it up and try it but my guess is I will want to put it back to stock. We shall see but gotta try it.
I know I did not ask a lot of questions on this (My 2nd Build) build that's because of all the previous posts you all have made and Dougs information Library for this build had everything I needed to know just had to find it and research it.
I do have a few questions though if some of you could please reply.
1. I am going to order a head cab (I don't have the time and tools to build one working out of town right now) and have researched several. So far I am leaning towards Mojo. Any suggestions I think I want to get the smaller authentic looking head box. Maybe there is someone on the forum that builds quality cabinets? I ordered the face plate from E-bay and it is plastic were the originals metal? A friend that is a sheet metal worker broke the chassis for me and we made it 16 inches long which I believe is the correct size for this build.
2. Please look at my voltages and comment. I am really not sure on this I am able to read the schematic's but am not that deep into theory yet. Although I have been studying because I want to learn. The link to the voltage chart is in the 12/3 post I made.
Thanks Steve
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That's a very clean looking build. Nice work.
I especially like the 'fender' jewel on a 'marshall' design :wink:
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yeah, very nice, very clean build
You may just want to leave the switch on full time once you hear how it beefs up the sound.
Even the clean sounds are nicer, IMO
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Thanks for the comments
Doug my parts for the mod should be here today I am going to try it and see. I have been playing it for at least 2hrs a night since i finished it and it seems to sound better every time I play. Stuff must be breaking in now. I am really digging the mixed jumped channels but I am anxious to try out the mod.
Ok no answers on the cabinet I will post in cabinets. No comments on voltages so I assume I am good.