Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SILVERGUN on January 24, 2015, 10:22:30 pm
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I have a guy who wants a great harmonica amp, that would also work well for guitar...
We're on a low budget.
So I pieced this together from the 6G6B bass channel input, with a single gain stage into a cathode follower driven TMB.
To get him from harp to guitar I made the mid cap switchable from a front panel 6 pos. rotary switch.
In the 'Harp ranges' of that switch I used much smaller values than stock so as to bring out more mids and hopefully allow the harp to cut the mix. It also gave it a nice nasally honk, that when combined with the reverb,,,it immediately sounded Chicago Blues to me, and as he laid into the harp it only got better.....mission accomplished!
At full counter clockwise pos. of the switch he's in harp-land, and then I gave him clockwise steps down to start cutting some mids and get closer to typical Fendery cleans. (similar to the switch that sluckey and Ed made optional on the AC-15)
I put V1 right by the input again, and ran the input stage into the CF right there with terminal strips off to a separate tonestack board right behind the pots.
PI trimmer is stolen from Santa Cruz Ca. and no one can find the owner so it's all mine now....it does a cool job of unbalancing things for a nice 'thickening' effect that I set by ear.
I had a 120 to 230VAC .5 amp isolation xfmr that I had wanted to try out as a mains PT,,,so I did....wound up with around 260VAC ......and that caused me to have to use a separate filament transformer, so I did.
340 plates, 250 screens makes for only about 10 or so watts out of a pair of 6L6s :icon_biggrin: (originally thought I was gonna use 6V6 but the L's just took the job)
And with low drive coming from the P.I it is a very quiet amp that never gets too loud with guitar.
One design goal of a harp amp is to minimize gain and hence, eliminate feedback issues....let the big signal coming from the Green Bullet mic push the amp into overdrive......it does.
A single coil guitar does not....just stays clean, and falls short of Deluxe reverb land. Oh well, I'm on budget.
Added a phase switch on the speaker out to hopefully be able to counteract feedback on stage.
The cabinet was left over from a Judybox amp I had trash picked off of Ebay, with the 12" Bad Betty speaker still in it.....so it stayed. I took this around for some of my R+D guys and it got a big thumbs up and was referred to as "toneful"......I agreed.
More pics to come.....
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:bravo1: Very nice!!!!!!
Brad :icon_biggrin:
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:bump1: :bravo1:
(http://www.diyitalia.eu/forum/images/smilies/salutations-489.gif)
Franco
p.s.: if possible I would like to hear a small harp sample
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Very nice and well thought out. I've always been fond of putting the preamp right at the input jacks. Sunn did that. Just makes so much sense. I see you're getting quite good with Visio too. That's always been a fun part of amp building process for me.
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Nicely done! Like the way you cattywhumped the 1 string of caps to make 'em fit on the strip too! :bravo1:
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THANKS guys!
This came together pretty quick in December and the guy is already gigging with the amp,,,but I'll try to catch up with him and get a clip.....that has to happen anyway because the negotiations for sale included a free guitar lesson. :icon_biggrin:
I also need to snap a pic of it mounted in the combo, because I thought I did, but I can only find this blurry shot from behind. :BangHead:
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Very nice and well thought out. I've always been fond of putting the preamp right at the input jacks. Sunn did that. Just makes so much sense. I see you're getting quite good with Visio too. That's always been a fun part of amp building process for me.
Thanks!,,,look close, and you'll notice at least one of your construction techniques in there... my new favorite: the main ground "bar" made from a looped piece of 16 gauge buss wire and bolted at both ends, as a very convenient central ground location for all of the big stuff.
I wish I was good at, or even owned Visio...
That is all still cut and paste of your schematics using just Paint ...what a time consuming, un-fun part of the process for me.
Some days I feel like it and some days it's a chore, which is why I don't think I ever finished a schemo for the TDR.
Your documentation process is as impressive as your build quality and it is a bar that I am slowly reaching for. Any one who has tried knows how time consuming this all is.....if you can manage to keep the wife happy AND get stuff built, and get through the holidays, that's a major accomplishment.
:occasion14:
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Nicely done! Like the way you cattywhumped the 1 string of caps to make 'em fit on the strip too! :bravo1:
Thanks buddy.....I now have a new vocabulary word!
I cattywhumped those coupling caps that way and fed the 'lower' one with an under-board jumper from the PI balance pot.
It is fun to try new layout 'work-arounds',,,but I worry that it makes it look amateurish :icon_biggrin:
Didn't worry too long, cause I just needed to get it done while the guy had the cash.
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Didn't worry too long, cause I just needed to get it done while the guy had the cash.
:laugh:
I just finished my second little harp/geetar, and it sounds great.... except for the static I get on some notes. First I'll swap some tubes, then start hunting for a bad joint or resistor. I hope it's a resistor instead of sloppy soldering. :icon_biggrin:
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Nice work.
Had no idea Judy amps were made in Austin TX. Any idea what happened to them?
--Pete
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Nice amp! I like the idea of putting V1 right at the input jack and the ground bus has worked well for me too.
When I had a workshop, I built a Super Reverb with the "Normal" channel's Bright switch turned into a guitar/harp switch.
(http://i222.photobucket.com/albums/dd298/Chipster457/Super%20Reverb/UberVerbSchematic-Preamp.gif)
You'll notice that I boogered up the input jacks so that the Lo input has a much lower input impedance. It seemed to play better with a harp mic that way.
A lot of harp amps seem to use 220K plate and 2.7K cathode resistors. Sounded nicer to my ear with the harp and the amp's owner and I both like it in guitar mode too.
Cheers,
Chip
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Nice work.
Had no idea Judy amps were made in Austin TX. Any idea what happened to them?
--Pete
All I know is what I read here:
http://ampgarage.com/forum/viewtopic.php?t=25125&start=15&postdays=0&postorder=asc&highlight (http://ampgarage.com/forum/viewtopic.php?t=25125&start=15&postdays=0&postorder=asc&highlight)=
And a little bit more by Googleing Judybox Revival...
Cliff's notes:
- Al designed a cool amp
- Al made some by hand and dude's liked 'em
- Al took on investors and moved some mfg. to China
- Al's amps suffered because quality was gone
- Al got sick and wasn't the owner anymore, but still did service and pissed a bunch of amp owners off by keeping their repairs for months/years
- Company folded
I saw first hand the lack of quality.... :huh:
I got the cab, with chassis and xfmrs, pots and switches, and just the heaters wired.....no board = $80
Besides all the components being crap the cabinet was coming apart.
Turns out that the speaker baffle was only held in by some glue where it butted up to the sides of the cab,,,so when the guy sold it to me he showed me how the speaker would just kinda rock back and forth.....today it's solid as a rock.
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When I had a workshop, I built a Super Reverb with the "Normal" channel's Bright switch turned into a guitar/harp switch.
Cool...thanks for sharing that.
This was another step in the learning process, and I based some of my decisions on an article by Gerald Weber.
http://www.jt30.com/jt30page/weber/ (http://www.jt30.com/jt30page/weber/)
One of the tips I didn't follow was using a tube rec....so I used 2.2K screen resistors as 'simulated sag'
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Nice work!
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Your PI pot remembered me this thing (based on KOC The Ultimate Tone 5)
http://www.mediafire.com/download/zjicp230r2m2tto/SOMA84.pdf (http://www.mediafire.com/download/zjicp230r2m2tto/SOMA84.pdf)
http://www.mediafire.com/download/4zoa7441kkrsr4j/PP-SE_amp_sample.mp3 (http://www.mediafire.com/download/4zoa7441kkrsr4j/PP-SE_amp_sample.mp3)
Franco
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from Silvergun:
"That is all still cut and paste of your schematics using just Paint ...what a time consuming, un-fun part of the process for me."
I tried that on my current project and it sucks, so I went to the web ordered drafting templates and I'm going old-skool! Think I might even draft the layout on canvas!
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Your PI pot remembered me this thing (based on KOC The Ultimate Tone 5)
Thanks K
I downloaded and saved that pdf....
This amp didn't have that much drive with a guitar input.
from Silvergun:
"That is all still cut and paste of your schematics using just Paint ...what a time consuming, un-fun part of the process for me."
I tried that on my current project and it sucks, so I went to the web ordered drafting templates and I'm going old-skool! Think I might even draft the layout on canvas!
I like it....that does sound like fun.
It never even crossed my mind to try to make a nice drawing by hand,,,,most of my renderings look like drunk chicken scratch.
Eor me,,,committing to one program is the biggest sticking point......I promised tubenit (a Long time ago) that I would get better at express SCH, but everytime I go to edit a schematic I open Paint
I need to get a copy of Visio and lock myself in a room with it,,,,but life is in the way.
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I'm getting good at expess sch, but paint!!! and visio is outta my range now. Nice build also, the bar is high here and that is WHY I stay. Mediocrity can be had anywhere. Is the chassis one of dougs stouts?
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Really nice work and amp - sounds like it's a kick to play harp through. :icon_biggrin:
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Is the chassis one of dougs stouts?
No...but it would be most similar to his 17x6.5x2.5 blank chassis, but mine has the lips folded in.
I got from DirtyDawg before his shed burnt down :sad:
It is 17x8x2 and I choose it simply because it was laying around here and I need a quick fix...
I used the original judybox chassis for the last build I posted (the Rhinol TDR head), and luckily this was the same size!
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Really nice work and amp - sounds like it's a kick to play harp through. :icon_biggrin:
Thank you sir.
Yes, I judged success by the look on his face. :smiley:
I'll try to get him to do a clip.
The coolest part of this project for me, was that I had a "design" objective, and was able to make the amp what it needed to be for the application.
That felt like a big win. Of course, I used the information that is laid out here for all of us,,,and researched "the best harp amps", "making a great harp amp"
That's where I found the Weber article. Then I pieced it together from there.
I also saw where tubenit had raised the value of the entrance cap to the reverb circuit (in one of his designs), and I think that contributed to getting more wet mids, and an overall fatter reverb for harp. (so .001 instead of a stock 500p in the AB763 reverb circuit, feeding the reverb driver)
Combining that with some larger coupling caps throughout seemed to get it right.
The happy accident here is that I found a couple of positions on the mid switch that sounded cool on guitar that I might not have otherwise tried.
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Nice work.
Had no idea Judy amps were made in Austin TX. Any idea what happened to them?
--Pete
All I know is what I read here:
http://ampgarage.com/forum/viewtopic.php?t=25125&start=15&postdays=0&postorder=asc&highlight (http://ampgarage.com/forum/viewtopic.php?t=25125&start=15&postdays=0&postorder=asc&highlight)=
And a little bit more by Googleing Judybox Revival...
Cliff's notes:
- Al designed a cool amp
- Al made some by hand and dude's liked 'em
- Al took on investors and moved some mfg. to China
- Al's amps suffered because quality was gone
- Al got sick and wasn't the owner anymore, but still did service and pissed a bunch of amp owners off by keeping their repairs for months/years
- Company folded
I saw first hand the lack of quality.... :huh:
I got the cab, with chassis and xfmrs, pots and switches, and just the heaters wired.....no board = $80
Besides all the components being crap the cabinet was coming apart.
Turns out that the speaker baffle was only held in by some glue where it butted up to the sides of the cab,,,so when the guy sold it to me he showed me how the speaker would just kinda rock back and forth.....today it's solid as a rock.
thanks for the info regarding yours. there's one for sale locally and why my interest was piqued. glad i didn't pull the trigger on it. seemed like too good of a deal for 350USD.
http://austin.craigslist.org/msg/4802187183.html (http://austin.craigslist.org/msg/4802187183.html)
--pete
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I read where they we're selling for $1300 new.....and that 's how some guys were getting pissed.
They didn't want a combo that was mostly assembled in China.
The soldering was the worst!
I'd pay $200 for that Craigslist'er just to have a carcass to rebuild.....no more than that.
Makes a nice harp amp. :icon_biggrin:
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Okay, just have to ask..the "phase " switch...what effect does it have on a single speaker? Thanks...
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Okay, just have to ask..the "phase " switch...what effect does it have on a single speaker? Thanks...
This is from the Weber article mentioned above:
"Here�s the other part of the accident.� I found if I connected the speaker reverse polarity with respect to which half of the waveform is bigger, then the threshold of feedback goes up.� This means the amp can be turn up to ridiculous volume levels before acoustic feedback occurs!� I decided to leave the potentiometer as a �waveform symmetry� control so Paul could fatten his tone while reducing feedback!� "
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I'm sorry to be dense...I've read GW's article...if there is only a single speaker, what is it out of phase with? And, does it actually have a discernible audible effect in your build? Again, with all due respect...
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...if there is only a single speaker, what is it out of phase with?
I remember reading that article(s) in Vintage Guitar mag years ago when it/they came out, I had a subscription, and it caught my attention.
This is the way I understand it, I may be wrong.
He, (GW) is talking about the speakers 'throw'. Which way the speaker moves 1st. A speaker can move forward 1st or backwards 1st.
You can have an amp (whether, SS or tube) where it's said to be 'in phase with itself', ie, non-inverting, from input to output.
Meaning that if a sine wave is sent into the amp with the positive 1/2 (top 1/2) of the sine wave 1st, then at it's output the same positive 1/2 (top 1/2) appears, then the amp, as a whole, from input to output is not inverting the signal (sine wave) then it's in phase with itself or non-inverting. Remember every gain stage inverts the signal 180 degrees. (CF are not a gain stage and do not invert the signal.)
(There's a Marshall amp that is wired with a Hi gain jack and a Lo gain jack, that adds a 12AX7 triode for the Hi gain, which makes the 2 different input jacks inverted from each other at the output, same with a # of Fender BF reverb amps.)
If you take such an amp, non-inverting, and hook up the positive speaker lead to the positive OT lead, then the speaker will 'throw' forward 1st.
There's another part of the article where he (GW) starts out talking about flip/flopping the speaker leads, positive/ground, on an amp they were blind testing with a harp player and he said that each time the speaker was wired reverse polarity (his term) or backward throw 1st, the harp player liked it much better. He said it was 'softer' sounding (not volume, aggressiveness) something like that.
So, I believe there's more to it and he (GW) was tying to try 4 points together (some of which, SG hasn't posted) when he say's, "Here's the other part of the accident."
1. Reverse throw of the speaker, for a softer sound, less presence.
2. He put in a pot to dial in the PI's bias.
3. Found they liked the sound of an unbalanced PI.
4. They say it lessened feed back which allowed the harp player to turn up louder.
Now whether anyone can hear if a speaker is wired forward or backwards throw 1st? Or if it makes a difference with an adjustable PI balance control with the speaker wired reverse 'throw', you'd have to try it for yourself. :dontknow:
I think he worded what he (GW) said when he wrote the article in a confusing way and could have been much clearer, but at the time he was making that amp for harp players to buy, so........ :dontknow:
Also I think what GW calls a 'waveform symmetry control' is the same thing as in Kevin O'Connor's TUT books, as a PP to SE control?
Doug has GW's 5th book that has a chapter on harp amps that may have more clear info on it, for sale in his web store. :dontknow:
Brad :think1:
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Willabe, thanks for your time.I think I have that VG here somewhere, I'm going to have to take a look-see. Silvergun, did you find an audible difference using the phase switch?
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Your welcome. :icon_biggrin:
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I don't play harp, but I have recorded those guys. They always showed up with their own rig and they were going to record through that rig or they weren't going to record at all. I wonder if polarity was an important factor in their rigs.
They seem to like moderate tube distortion where one side of the signal is still a good sine wave and the other is all mashed down and distorted. Maybe it makes a difference which side is compressing air and which side is decompressing. If I saved any raw tracks (and I was flush with free time), I could ascertain which way the speaker was moving for each side of the signal. It would then be interesting to do a statistical analysis of favorite harp amps to see what the polarity of the speaker is in relation to the signal.
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Silvergun, did you find an audible difference using the phase switch?
Most definitely...
I purposefully unbalanced the PI so that there was a noticeable difference in the polarity setting of that switch.
I don't know the science of it,,,but it does work.
And conversely,,,if you or I wanted to increase the potential for positive feedback, we would do the opposite, and align the polarity of the speaker with the larger waveform that we create by unbalancing the signal coming out of the PI....
I'm now assuming that this is what is happening in the Dumble ODS when you use the PI trim pot to maximize feedback in the amp.
Pretty cool stuff, that might have taken me longer to figure out if I hadn't tried this and read that Weber article.
Willabe,
The first part of the 'accident' was the PI balance pot that he used to unbalance the signal and thicken up the signal. (in my schematic)...So you can make this all happen on purpose.
I also put a PI trimmer in my new Bossman 100 and this has helped me to understand what I'm doing with it. :icon_biggrin:
:thumbsup:
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The first part of the 'accident' was the PI balance pot that he used to unbalance the signal and thicken up the signal. (in my schematic)...So you can make this all happen on purpose.
Sorta, he was trying to balance the PI, self balancing paraphase (his term), with a pot by ear. The accident was that they found they liked the tone better un-balanced.
It's in 'Tube Guitar Amplifier Essentials' green cover, 4th(?) book, section XI, 'Harp Amp Secrets and Tips'.
Brad :icon_biggrin:
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The first part of the 'accident' was the PI balance pot that he used to unbalance the signal and thicken up the signal. (in my schematic)...So you can make this all happen on purpose.
Sorta, he was trying to balance the PI, self balancing paraphase (his term), with a pot by ear. The accident was that they found they liked the tone better un-balanced.
It's in 'Tube Guitar Amplifier Essentials' green cover, 4th(?) book, section XI, 'Harp Amp Secrets and Tips'.
Brad :icon_biggrin:
doug's stout has an unbalanced PI. part of the tone magic. ;-)
--pete
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Hi Pete
which are the components that make you say that the PI is not balanced ?
http://el34world.com/Hoffman/files/Hoffman_Stout.pdf
K
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SG that is an awesome build! Just threw it together huh? You are too humble dude, there was a LOT of thought in this one.
Very nice job! :worthy1:
Jim
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Hi Pete
which are the components that make you say that the PI is not balanced ?
http://el34world.com/Hoffman/files/Hoffman_Stout.pdf
K
in the link above, he fixed it in the latest version(s) when adding the NFB loop. the original plan had no NFB and the grid bypass cap was connected to the top of the tail rather than ground.
link to the original stout schematic.
http://el34world.com/projects/images/HoffmanStout18watt_No%20Switch_Schematic.GIF (http://el34world.com/projects/images/HoffmanStout18watt_No%20Switch_Schematic.GIF)
--pete
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I see, thanks Pete
K