oscilloscope turned guitar amp?

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#21
you're very right, this is extremely helpful to me. thanks to everybody who's posted here. 🙂

i play an old les paul with double-wound humbuckers, and i'd like a warmer, tubey, blusey sound. i'm set on single-ended (could that be a pun? :nod: ), since this is my first build and i'd rather not complicate the design any further. i think i'll redesign around an EL34 (i like the slightly distorted marshall sound).

i have a question though: i've connected the first part of the heater supply, without the HT supply and without grounding the heater center-tap. all tubes show working heaters except for the 12AU7, which barely glows, if at all. the tube is connected as shown in sec1-supply.png. any thoughts? do i need to go ahead and finish the supply before assuming a fault somewhere?

thanks again for all the help,
~ brad.
 
#26
geekysuavo said:
i've reworked the poweramp stage around an EL34. sorry for the low picture quality.
http://files.geekysuavo.org/golem/powerv2.jpg

anything seem out of sorts?
~ brad.
Click to expand...
Brad-
Most of the 'bias charts' around seem to recommend about 70mA@300v, so you may be a bit 'hot' at 100mA.
Bias Charts

Do you already have the output transformer in hand? Impedance may drive your choice of output tube.


re: 12 AU7 heater:Is this a known good tube? You could swap in another tube from the same family (AX7, AY,AT etc) and check the socket wiring if you don't have a spare AU7.

Cheers
John
 
#27
no, i don't have the OPT yet, so i can change that. increasing the primary would lower the bias current... but the Ik(max) in the EL34 datasheet is 130mA and the loadline is well within the Pa(max) curve and Va(max) lines... would the tube really be running hot? 😕

re: the 12AU7. it worked before i pulled it out of the 'scope. i'm going to buy a new one tomorrow most likely, so we should see how that turns out. :xeye:

thanks,
~ brad.
 
#28
geekysuavo said:
no, i don't have the OPT yet, so i can change that. increasing the primary would lower the bias current... but the Ik(max) in the EL34 datasheet is 130mA and the loadline is well within the Pa(max) curve and Va(max) lines... would the tube really be running hot? 😕
Click to expand...
Brad-
re: OPT primary- I was thinking that if you already had a 5k OPT, a 6L6(or6V6,6BQ5) might be a better match since most (guitar amp) recommendations for the EL34 are around 3K.

re: Plate dissipation and bias current- I'm lazy and generally use calculators online for that sort of thing, but you may be on track with your calculations.

Weber Power and bias calculator

At $$ apiece, you want to take care of those EL34s. (I like 6V6s a lot, especially since Russian equivalents are cheap and available. Lots of 6V6GTs can be scavenged from old TV/HiFi/radio chassis as well. However, not as much power as you want, I guess)

Cheers
John
 
#29
6L6 vs. KT66

Brad,

Sorry I took so long to reply to your question. I really would urge you to use the 6L6GC over a KT66 because the 6L6 is way more common and can be much less expensive to replace. Beware that there are tubes being sold claimed to be KT-66's that clearly are not.

The 6CA7 is also a good choice IMO (Trainer and Marshall certainly thought so), and can be had for about $12 a copy in cheap Chinese versions which I would not hesitate to use BTW. Shuguang makes a good tube IMO for the money.

You had a rather high (over 1 meg ohm) control grid bleeder R listed on your revised schematic. That will probably result in the output tube running away from G1 current and hot plating. I recommend changing that to a 470k or 330 k ohm, 1/2 watt R. The grid coupling cap can be increased to 0.05 or 0.1 uF.

This was mentioned by another and I will restate it. The output xfmer used in Hi-Fi is designed for lowest distortion as would the Edcor unit you showed as your choice. For guitar the output xfmer is usually undersized purposely and also contains other 'tricks' to make it distort in a deliberate and desireable way. If you can, get a real guitar amp o/p xfmer. There don't seem to be many made however for SE applications. Most are P-P. I saw that the Fender Champ is a small 6V6 SE amp but that xfmer is only rated at about 5 watts. Might be just what you want though.

For what you are trying to do, I wouldn't hesitate to try larger sized vertical output xfmers from vintage B&W TV sets as the output xfmer. Some are a true two winding xfmer and are the one you want. The undesireable one is the autotransformer with 3 wires. This would work but would put B+ on your speaker terminals creating a real safety hazard and eliminating the option of putting a remote speaker jack on your amp. Vertical OPT's could be had for free and I've seen them as large or larger than the Fender Champ xfmer.
 
#31
Re: 6L6 vs. KT66

ok,

rcavictim said:
You had a rather high (over 1 meg ohm) control grid bleeder R listed on your revised schematic. That will probably result in the output tube running away from G1 current and hot plating. I recommend changing that to a 470k or 330 k ohm, 1/2 watt R. The grid coupling cap can be increased to 0.05 or 0.1 uF.
Click to expand...

thanks for that advice. so i can replace the 1.15M resistor with a 330k and the 25nF capacitor with a 0.1uF. sound ok?

rcavictim said:
This was mentioned by another and I will restate it. The output xfmer used in Hi-Fi is designed for lowest distortion as would the Edcor unit you showed as your choice. For guitar the output xfmer is usually undersized purposely and also contains other 'tricks' to make it distort in a deliberate and desireable way. If you can, get a real guitar amp o/p xfmer. There don't seem to be many made however for SE applications. Most are P-P. I saw that the Fender Champ is a small 6V6 SE amp but that xfmer is only rated at about 5 watts. Might be just what you want though.
Click to expand...

how about these?
EDCOR GXSE10-8-5K
Doberman Amps custom

thanks!
~ brad.
 
#32
geekysuavo said:
ok, so you're recommending a lower output transformer primary impedance and a decreased cathode current? the only solution i can see to that is to decrease the HT voltage, which is why i chose a larger tube in the first place...

i'm a bit flummoxed... :scratch:

thanks,
~ brad.
Click to expand...


I did not recommend a reduced op tube current. That is set by the bias which is created by the tube current flowing through the value of the cathode resistor to ground. I was talking about reducing the value of the R between G1 and ground. It drains off the DC voltage that the grid develops by itself which can pull the grid positive (defeating the desired negative bias) and cause the tube to 'run away' and redplate. A lower 'grid leak' resistor value prevents this.

As for an op xfmer. Getting one for free from an old TV or a TV shop and trying it, even if the results are not to your liking won't cause any harm. I suggest it because there is a good chance it will work to your satisfaction and save you $30-$50 cash. Forget math and transformer impedance calculations with this experiment(Z). Just try it. 😉 This suggestion is made in the spirit of this being a real budget, low power DIY starter guitar practise amplifier.
 
#33
rcavictim said:
I did not recommend a reduced op tube current. That is set by the bias which is created by the tube current flowing through the value of the cathode resistor to ground. I was talking about reducing the value of the R between G1 and ground. It drains off the DC voltage that the grid develops by itself which can pull the grid positive (defeating the desired negative bias) and cause the tube to 'run away' and redplate. A lower 'grid leak' resistor value prevents this.
Click to expand...

sorry, my comment was related to an earlier line of discussion. as for an old tv, i'll look into it. sounds kind of exciting! 🙂

thanks again for all the help,
~ brad.
 
#34
Re: Re: 6L6 vs. KT66

rcavictim said:

If you can, get a real guitar amp o/p xfmer. There don't seem to be many made however for SE applications. Most are P-P.
Click to expand...
As Bob says, you can get away with a lot when it's a guitar amp- I've had good luck with small OPTs from old radios and TVs- though if you are putting 100mA through something on the small side it may overheat. It never hurts to try.
I've had good luck with guitar OPTs from here .

geekysuavo said:
ok, so you're recommending a lower output transformer primary impedance and a decreased cathode current? the only solution i can see to that is to decrease the HT voltage, which is why i chose a larger tube in the first place...
Click to expand...
I think you want to look at the cathode (bias) resistor value- get a few on hand to do some experimenting- for controlling the current through the tube (and plate dissipation). If you can get your hands on a large-wattage rheostat or adjustable power resistor you can find the desired Rk value when you breadboard the amp.


geekysuavo said:


how about these?
EDCOR GXSE10-8-5K
Doberman Amps custom
Click to expand...

The Doberman/Heyboers are a bit expensive for my taste and for this sort of project. In the Edcors, I'd pay a few extra bucks for the multi-tap secondaries (if available-apparently Edcor has a lot of stuff that is not listed in the online catalog, so I'd call) since you have a lot more possibilities for :
a) playing with different speaker combos
b) 'mis-wiring' the speaker to the 'wrong' tap to change the impedance the tube 'sees'.
5k for 6L6 and 3k for 6CA7/EL34
Even something from the XSE line would be fine for a guitar amp, IMO.

Cheers
John
 
#39
after multiple headaches...

i've revised the entire set of schematics, and here they are. the power supply is built and works 100%. i've yet to build any of the other stages, as i don't have all the parts here.

power supply v2
preamplifier v2
power amplifier

REVISIONS: i've omitted the tone stack and added a gain knob to the power amplifier, since i really didn't have the right valued components to make a tone stack that worked. the 12AU7 triodes are both cathode biased now, since i have no suitable LEDs and to preserve the vintage/scrapheap 🙂 quality of the amp. the standby SPST switches the HV center-tap to ground for safety, and the 100 ohm resistors were removed since they served no purpose. the component names come from the EICO 460 manual schematic.

thanks for all the help; i couldn't have come this far without it!
~ brad.
 
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