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Guitar amp for Wes is running!
Guitar amp for Wes is running!
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Old 11th May 2008, 10:26 PM   #1
mashaffer is online now mashaffer  United States
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Default Guitar amp for Wes is running!

Some of you may remember that I was working on a guitar amp for my grandson. Well it is up and running in beta test version. I am very excited about getting it this far. I now have till the 18th to get it tweaked and the initial voicing done.

Did the voltage tests over two nights.

Saturday:
The first thing that I discovered is that the switches on most of the 1/4 jacks were not working (always open). I examined them and discovered that the spring pressure had moved the contacts upwards in the plastic body. I suspect that over zealous soldering softened the plastic enough to damage the locking mechanism. So I used a pair of needle nose pliers to put them back in position and put a dab of super glue on in hopes that this will keep them in place. In the long run I may have to replace them. Anyone else run into this problem before?

When I got to the part that involved plugging it in things went a bit more smoothly. The voltages out of the transformer all fell in line with expectations so I installed the recto and the standby switch worked as desired, bleed resistor was working and no caps exploded.

I proceeded with installation of the signal tubes and fired up again. The parts that were supposed to glow did and the ones that were not supposed to glow didn't. So I went on to measure voltages starting at the power tubes (the theory being that if there was excessive current in the power tubes I wanted to know about it ASAP) and I got as far as measuring the PI tubes before I quit for the night. The results are

371V - 6V6 Plates
320V - 6V6 Screens
21V - 6V6 Cathodes

306V - B+ for PI
217V - PI Plates
40V - PI Cathodes
30V -PI Grids

290V - B+ for final gain
283B - B+ for input stages

With both input stages and the MV at 50% I could hear a little bit of noise like faint rushing water and maybe a touch of hum with my head about one or two feet from the un mounted test speaker.

Sunday:
Well it didn't take long to get the rest of the voltages. They are as follows.

CF:
290V Plate
111V Cathode 100k
106V Grid

Final Gain:
106V Plate 100k
1.4V Cathode 820R/0.68uF

Normal Channel:
167V Plate 100k
1.1V Cathode 820R/330uF

Bright Channel:
205V Plate 100k
2.2V Cathode 2.4K/0.68uF

BTW, schematic is available at http://tubbythetubie.embarqspace.com/ under the "Guitar Amp Project" page.

Since there was no stray DC on the input jack I decided to hook it up to my keyboard (don't have a guitar) and it did make music. The normal channel was very full and the bright channel was very bright so that is as it should be.

I was able to get some fuzzy distortion with the keyboard output and the preamp gain controls set high enough. With controls set lower the sound was very clean.

With the cascade switch on of course the tone was dominated by the bright channel but the gain was quite noticeably higher and it was pretty easy to get the fuzz without blowing the eardrums out.

I was doing the tests using the Bad Betty (93dB) speaker as I wasn't yet confident enough to risk the Man-O-War.

So all in all functional but there were a few glitches that need to be tweaked.

1. There is a little bit more noise than I like. Hiss and something like a buzz is clearly audible at normal playing volume though not deafening. Fortunately there is no hum. I only used shielded cable on the input to first tube grids and used regular wire for all other connections such as gain and tone controls. I could try going to shielded in more of the signal path.

2. The treble and mid controls work as expected but not as much difference as I expected. Maybe using a guitar will make it more obvious. The Bass however seems to make much less difference, works backwards and seems to have most of its effect in the very first few degrees of travel. I though sure that I had wired it correctly but I may have a couple of wires crossed. It acts like there is a linear pot in there so I will have to recheck and make sure it is log.

Any other things to check in this regard. Could a loading issue be the culprit.

3. The gain and MV pots have a little glitch at the very bottom of their travel just as it comes in. This may just be a feature of the pots and should not be a problem as long as the behavior is confined to the turn on area. The pots are conductive plastic.

A friend of my granddaughter here in town has volunteered his services to test it out so will know more when he gets his guitar on it.

Advice welcome.

mike
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Old 12th May 2008, 12:25 PM   #2
Trout is offline Trout  United States
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It appears you are still over your bandwidth,

I always get this message when I click for the PDF.

Download Error!

This site has gone over its daily download limit



Please try again later
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Old 12th May 2008, 11:24 PM   #3
mashaffer is online now mashaffer  United States
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Hmmm... must have generated some interest. I have neve had enough traffic to even realize there was a limit.

mike
Attached Files
File Type: pdf wesamp.pdf (28.5 KB, 70 views)
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Old 14th May 2008, 03:48 AM   #4
Trout is offline Trout  United States
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Quote:
2. The treble and mid controls work as expected but not as much difference as I expected. Maybe using a guitar will make it more obvious. The Bass however seems to make much less difference, works backwards and seems to have most of its effect in the very first few degrees of travel. I though sure that I had wired it correctly but I may have a couple of wires crossed. It acts like there is a linear pot in there so I will have to recheck and make sure it is log.
One thing I noticed is the wiper in the schematic is on the wrong side of the pot based on others like the Fender Bassman which uses this common tone stack.

Poor resolution near the bottom of the gain and master volume pots is pretty common, I see this a heck of a lot with alpha pots time to time.

Quote:
The first thing that I discovered is that the switches on most of the 1/4 jacks were not working (always open). I examined them and discovered that the spring pressure had moved the contacts upwards in the plastic body. I suspect that over zealous soldering softened the plastic enough to damage the locking mechanism.
I have had this happen several times with the Neutrik brand jacks.
Though similar to Cliff Jacks, the plastic compound is not very heat resistant, Even a light touch soldering and they loosen up pretty bad.
Switch to the Marshall style Cliff branded jacks will help this or just use the Switchcraft jacks like the old fender amps.

I suspect the 500PF bright cap is a bit extreme (for my taste) I rarely use over 250pf and often only 100pf. That might be contributing to the hiss you mention.
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Old 15th May 2008, 05:29 PM   #5
mashaffer is online now mashaffer  United States
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Thanks Trout. I have some old switchcraft style jacks in my salvage bin so I am replacing the plastic with these. I am one short so will have to pick up another for the input (though so far it is not giving me any trouble)

When using the metal jacks would you recommend insulating them from the chassis. Right now the one that I have installed is not insulated but I am still running a dedicated ground wire to the signal ground point.

I have a guitar player coming over today to test so I will see if the bright sounds excessive. You are talking about C11 and 12 right?

I did notice last night that the noise is quite reasonable when nothing is connected to the input so it may be something in the keyboard or the cable that I am connecting with that is responsible for the bulk of the noise. After todays testing we will have a better idea.

As to the bass pot wiring, you are right! I drew the schematic wrong and then wired it according to the schematic.

Pics are posted here...

http://www.hawthorneaudio.com/forums...p=29684#p29684

mike
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Old 15th May 2008, 05:36 PM   #6
Nordic is offline Nordic  South Africa
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Nice work.... Some tolex, some chipboard (to keep the beer out) and a faceplate and you have a realy nice amp head there...
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Old 15th May 2008, 08:54 PM   #7
mashaffer is online now mashaffer  United States
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Default Test report

As promised a short report on the results.

Jake brought one of these http://www.musiciansfriend.com/produ...ium?sku=511117 a couple of cables and one of those digital multi-function pedals (sorry I didn't get which one). We ran the guitfiddle into the pedal and then into my amp. We started with the output hooked up to the $25 surplus speaker (Bad Betty) until I convinced myself that it was safe to try the more expensive Eminence Man-O-War.

The noise level was much lower than when my keyboard was hooked up to it; very pleased with that. We got very loud levels before the hiss really began to show up. Switching over to the MOW both I and the guitar players felt that it had the better tone as well as being 10dB loader. It actually had really nice bass so I was pleased with that.

Without pedal distortion we were able to produce some preamp distortion using just the amp and guitar controls but feedback was a limiting factor here. We were at ear splitting levels but right on the edge of feedback. I had noticed that the tubes were showing some microphonics when turned up pretty loud (running finger along the chassis could be heard in the speaker) so I suspect that finding some less microphonic tubes might be of help here. I am not well schooled in the feedback mechanisms in guitars but it is possible that the tremendous volume levels involved may also have been exciting the strings and causing incipient feedback. Considering the possibility that direct electromagnetic coupling between circuits might be involved as well, I think it wise that I go ahead and change to shielded wire in as much of the signal chain as I can. This is especially so since with sufficient gain (cascade switch on) we were able to get feedback at more moderate volume levels.

The players also agreed that the bright channel was really brighter than it needed to be so changing those 500pF caps in the bright channel to 250pF is probably a good idea and should further reduce noise. Whether it might help the feedback also I don't know.

All in all I think the noise is acceptable and that it can be reduced somewhat. Feedback now seems to be the major issue to deal with. Reliability is of course an unknown quantity that will only be fully addressed with time.

Many thanks for all of the assistance and moral support.

mike

BTW, this will be a 1x12 combo so microphonics is probably a very important issue.
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Old 15th May 2008, 09:05 PM   #8
Nordic is offline Nordic  South Africa
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I did quite a few online comparisons before I got my 12ax7s as I almost had some EI ones on order...it would seem good choises for these are original Mullard EC83 and Electro Harmonics 12ax7EH, which is what I settled for.
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Old 15th May 2008, 09:43 PM   #9
mashaffer is online now mashaffer  United States
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Like these?

EH 12AX7s

mike
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Old 16th May 2008, 10:35 AM   #10
Nordic is offline Nordic  South Africa
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Yep, even the same box, cute fat little tubes... they realy fill the socket.
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