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Old 9th February 2005, 12:16 AM   #1
Trout is offline Trout  United States
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Default 6AQ5A guitar amp revisited

Welp,

After several hours of point to point nightmares, I have the amp IM working on semi-sucessfully running, WOoow no smoke either

Still have several issues needing worked out though. Basically I started with a rough version of This Drawing
Needless to say, the sound output was at a level so low, I had to hold the 12" speaker next to my ear!

I proceeded to remove the tone section as shown in
This drawing

Now I have what seems like 3-6 watts of VERY over driven sound, In fact Excessively over-driven. Ended up putting in a 12au7 to tame it.

SO, I removed the 100uf cap and changed the 15K resistor in preamp to 4.7K,, Much cleaner, Can now run the 12AX7a again without much problem. It still over-gains somewhat at full settings but manageable.

Base Issue: So far, Its seeming weak on low frequencies, I really thought I would see considerably better base response than IM Getting,

I removed the 2500 ohm primary opt and opted for the 5000 ohm opt fender replacement. Output level and base improved slightly BUT,, still lower than what I would consider normal.

Presence in mid range and Highs are incredible!!

IM also experiencing a low level hum at full output, which I was advised would most likely happen. Is it Better To UP the cap values? or add the choke? or switch to full wave solid state rectification?

Oh,, one more thing, Im thinking that the 500ohm bias is way to high on the 6AQ5a, Perhaps according to what I have read , It should be closer to 300 Ohm.

Thanks Guys !
Any Ideas or helpful
Gene
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Old 10th February 2005, 12:33 AM   #2
Trout is offline Trout  United States
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IM also experiencing a low level hum at full output, which I was advised would most likely happen. Is it Better To UP the cap values? or add the choke? or switch to full wave solid state rectification?

Oh,, one more thing, Im thinking that the 500ohm bias is way to high on the 6AQ5a, Perhaps according to what I have read , It should be closer to 300 Ohm.

Really hoped someone might have a few ideas/answers to the above questions.
So far
I have changed the bias resistor to 360 ohms. Really seemed to have no effect.
I removed the second 100 uf cap in the preamp stage and successfully cleaned up the distortion to now what is a very clean crisp setup. Presence is amazing Still lacking a little base but acceptable.

Im still getting enough gain that unwanted high frequency feedback occures at near full output settings.

Id really like to clean up the hum a little, One thing I had not shown in the original drawings was the CT on the 6.3v filamant leads. Its connected to nothing, Should that be grounded to the common ground?

I love to tinker,, Im smart enough to be dangerous , I just hope I nan get thru this without sniffing any burnt componets
Gene

I'll post an edit of the latest mods on the drawing later, Honeys callin me for dinner
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Old 10th February 2005, 04:04 PM   #3
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Gene,
Try moving the screen (pin 4) to the 1k/5uf node in the power supply. In a guitar amp, you want the screen to be almost the same voltage as the plate. Increase this cap to 47uf. After the 10k resistor, this cap should be 22uf, connect the preamp here. You don't need the last supply stage. Use a 220kohm or higher bleed resistor, and put it after the 100 ohm resistor. The present 100k wastes too much current.

Connect the 6.3v center tap to the 6AQ5 cathode, this often works better at reducing hum than connecting to ground. The 6AQ5 cathode resistor should be 270 - 330 if the plate voltage is around 250v.
If you still have hum problems, then you most likely have grounding issues and these can be difficult to track down.

Preamp suggestions:

1st stage go with the classic Leo Fender 100k plate / 1.5k cathode, bypassed with a 1uf - 22uf cap. Stick a 33-68k grid stopper on it , to help kill any RF that might be getting through.

2nd stage put a 220k grid stopper and try a 120k plate resistor and a 680 ohm cathode resistor bypassed with a 1uf cap. On this stage, you want the plate voltage to be about half the supply voltage, this will give you a very nice "plexi" type of overdrive sound, and plenty of "meat" in the clean sound.

The way you have it now, the preamp stages are running very little current, and that often sounds thin and buzzy and too easily overdriven into cutoff.

I'd put the tone controls back in, use the one on the second schematic. Even with the tone controls in, there should be more than enough signal voltage to seriously overdrive the 6AQ5.

Also, put a 1.5k or higher grid stopper on the 6AQ5. The grid stoppers help prevent "blocking distortion" during overdrive and help prevent oscillations, which can be ultrasonic, so you can't hear them but steal a lot of power and create unwanted intermodulation distortion and all kinds of other junk that you don't want.

have a look at the p1 extreme at www.ax84.com.

Regards,
Mike
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Old 10th February 2005, 05:09 PM   #4
Trout is offline Trout  United States
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Great Post Mike.

I have redrawn this baby again to exactly what Im running this second Junk Yard Amp

Now that I have a 12AU7a replacing the 12ax7 Its really sounding Sweet. Extremely clean sound,Very low parts count amp. Im Guessing around 4.5-6 watts Driving a Celestion G12K-85 speaker. Oddly, Volume output was not effected by swapping out the 12AX7. BUT,, What residual hum remaining is a level so low, Im thinking DC filaments might make it dead quiet!!
Also lost all the high frequency feedback at full gain setting.

Im running Gain And Master volume at FULL and hum is now well within the tolerable range.

The Tone of the guitar ( 78 american Strat w/ lace sensors )
Really shines. When switch thru the pickup complement, Each setting has a very distinct affect.

This setup clearly wouldnt be ideal for Base guitar though. I think the blend right now is about as perfect a clean setup as any I have heard, But , Then again, A guitar is more a midrange instrument.
Results, Fairly Clean Amp,, Let the stomp box do the rest
Gene
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Old 11th February 2005, 04:01 AM   #5
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Quote:
Originally posted by Trout

The Tone of the guitar ( 78 american Strat w/ lace sensors )
Really shines. When switch thru the pickup complement, Each setting has a very distinct affect.

This setup clearly wouldnt be ideal for Base guitar though. I think the blend right now is about as perfect a clean setup as any I have heard, But , Then again, A guitar is more a midrange instrument.
Results, Fairly Clean Amp,, Let the stomp box do the rest
Gene

After about 2 hours of playing, And pondering the possibility of even fuller sound, Ive made a few of Mikes mods as shown In This

JunkYard Drawing.

Still can not run the 12AX7A Tube,, Terrible high frequency feed back when gain control is raised near a crunch level.(above 50%)

But otherwise, When gain control is lowered enough, Its very clean and crisp. Just lacking volume.

12AU7a Still tames the beast, Even at near full gain setting ,Though you completely loose most the overdrive sound.

I am still kind of stumped on this feedback issue with the 12AX7, Seeing as its such a common preamp tube in a guitar amp.

gene
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Old 11th February 2005, 07:33 AM   #6
Giaime is offline Giaime  Italy
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Have you tried another 12AX7? It may be a defective/microphonic one...

Look at the wiring, most of the times hf oscillations is caused by running PSU wires near the input or something like that...

I'd suggest to you to remove the cap at the input, and to lower that grid stopper to 47k/68k like in most amps.
Also you might want to reduce the screen grid resistor, look at the tube datasheet for suggestions. And remove that 100k bleed resistor that's taming the voltages!

And about bass... most of the time the lack of bass in little amps is caused by the OT. But try to increase the cap at the 6AQ5 input... maybe to 0.1uF or something... If you do this you should also increase the 1.5k to the grid of the pentode... 5.6k sounds adequate.
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Old 11th February 2005, 01:59 PM   #7
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Gene,
The oscillation is almost always caused by the 2nd gain stage. Two things to try.
1. Put a grid stopper resistor directly on pin 7 of the 12A_7.
Start with 220k. Solder the resistor right on the pin with the shortest lead length possible. This should kill the oscillation. If not try 470k.
2. You may also need to use shielded wire from the wiper of the gain pot to the stopper resistor.

-Mike
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Old 11th February 2005, 03:18 PM   #8
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Quote:
Originally posted by Giaime
Have you tried another 12AX7? It may be a defective/microphonic one...

Look at the wiring, most of the times hf oscillations is caused by running PSU wires near the input or something like that...

I'd suggest to you to remove the cap at the input, and to lower that grid stopper to 47k/68k like in most amps.
Also you might want to reduce the screen grid resistor, look at the tube datasheet for suggestions. And remove that 100k bleed resistor that's taming the voltages!

And about bass... most of the time the lack of bass in little amps is caused by the OT. But try to increase the cap at the 6AQ5 input... maybe to 0.1uF or something... If you do this you should also increase the 1.5k to the grid of the pentode... 5.6k sounds adequate.
OK,
I removed the input cap,The Grid stopper was a typo, It is actually 33K.
I had thought of the defective tube possibility, I ran an RCA, A Mullard, And an EH, They all acted the same.

Changed the Cap at the 6AQ5 to .1 orange drop instead of the original .02 black beauty.
The Transformer : Most likely marginal

Single-ended output transformer with a 5000 ohm primary and both 4 and 8 ohm secondary connections.
A bolt-on replacement for Fender Champs(fits silverface & blackface Champs from 60's & 70's), Typically used with tubes such as 6BQ5/EL84, 6BM8/ECL82, 6AQ5/EL90, 6CM6, 6V6-GT.
Transformer is rated 100 Hz to 20 KHz -2 dB at 5 watts with 40 ma (maximum recommended) primary current. Reducing primary current improves the bandwidth ,because this is a single ended transformer.

SO, after making the above changes,The 12AX7 is still not working out, The 12AU7 approaches the high frequency feedback point at about 90% gain setting.(tolerable)
Mind you this is amp is now incredably Clean And Extremely Brite sounding otherwise .

I do have another THIS OPT But I have no clue how to squeeze it onto this tiny chassis (6 1/2 sq) unless its mounted remotely.
gene
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Old 11th February 2005, 03:27 PM   #9
Giaime is offline Giaime  Italy
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So, how does it sounds with the circuit changes I suggested you?
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Old 11th February 2005, 04:24 PM   #10
Trout is offline Trout  United States
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Quote:
Originally posted by Giaime
So, how does it sounds with the circuit changes I suggested you?

Sound???

With 12AU7a

Omg its Brilliant, Crisp, Very Full tone, Very little overdrive/distortion even at full gain setting.Base is still marginal(OT suspected).

Im going to try the grid stopper on the second prestage now.
Theres still some kind of high frequency thing going on. Its tamed somewhat by the 12AU7, Terrible with the 12AX7.

All signal leads are sheilded , and theres nothing more that a couple inches in length. I rechecked all the ground points , All jacks, pots,and chassis grounds are secure.

Solder time again

I'll post more results in an hour or so.

Thanks for all the help!!!!!!!!!
Gene
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