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cnludwig

says: Feb 27, 2011. 5:59 PM REPLY

Cool project. I like how you used otherwise unwanted parts (power tubes and transformer) and created a design around it.

gmoon

says: Feb 28, 2011. 6:00 AM REPLY

:-)

eridan

says: Dec 18, 2010. 12:08 AM REPLY

Very interesting. Thanx a lot!

smartrobot

says: Dec 3, 2010. 5:20 PM REPLY

This person had a bunch of boxes on the curb it said free so I took a look, and one of the boxes was full of vacuum tubes. some of them are not labeled so how can I find out what they are?

gmoon

says: Dec 4, 2010. 6:08 AM REPLY

Almost all tubes are marked in some way. Sometimes the labeling is hard to see, but try looking at different angles and varying light conditions.

Be careful when cleaning tubes; you can rub off the markings...

smartrobot

says: Dec 5, 2010. 2:25 PM REPLY

I googled the tubes that had letters on them it looks like they came out of a old tv so it does not matter.

katatonicdean

says: Dec 2, 2010. 4:29 PM REPLY

Alright, thanks for all your help!

gmoon

says: Dec 3, 2010. 12:42 PM REPLY

Antec--that was the company that makes a 150V toroidal transformer with filament windings too, at a reasonable price. It 's the AN-1T150 model.

Find their stuff here . I've never used it, but that's the closest I've seen to the transformer I used, and it's not very expensive.

katatonicdean

says: Dec 2, 2010. 1:28 AM REPLY

Hey,

First off, I was just wondering if this can be converted into a bass amp with an added bass knob.

Secondly, do you know of a place that I can get a similar choke, power transformer, and output transformer?

And thirdly, what pins were used for the tubes? It would be really helpful to know. (I notices that you used the 12ax7 twice, but on the power supply you only had one of the filament pins powered?)

Thanks for all your help!

gmoon

says: Dec 2, 2010. 12:55 PM REPLY

Converting to a bass amp? Probably by using somewhat lower value coupling capacitors. Where this amp uses 0.02 uF caps, you could go with 0.033 or 0.047 uF coupling caps.

That would be more "bassy". If you need a separate bass knob, then use a different t one stack in place of the existing one. This website will be helpful.

Edcor makes the output transformer. You can find similar voltage power transformers, but that's a bit harder. I had a nice link to a 150V toroidal PT, but I lost it when my computer died. If I find it I'll post it later. Chokes are easier, Hammond makes those.

You'll have to look up the pin outs yourself, they are on the datasheet I linked (and you can find these online)-- in fact, if you've never built a tube amp before, I'd recommend you do a lot more research. You'll figure it out...

The 12ax7 is listed twice, but it's a dual triode, so t wo triodes are enclosed in the same tube. Hence, only one filament is needed (note: 12ax7s can be wired as either 12.6V or 6.3V filaments).

katatonicdean

says: Dec 2, 2010. 1:48 AM REPLY

Oh, and can i use a higher ohm speaker. I was thinking an 8 ohm or 16 ohm speaker. What would I need to do?

gmoon

says: Dec 2, 2010. 12:42 PM REPLY Choose an output transformer with whatever i mpedance you need.

I.E., if you have a 16 ohm cabinet, get a transformer with a 16 ohm output impedance. I'm sure Edcor makes this trannie in both 8 and 16 ohm versions.

Some OTs have multiple coil taps. You'd need to special order that from Edcor...

ski4jesus

says: Oct 9, 2010. 4:49 PM REPLY

DUDE. that is sooo sweet haha hey btw, i have that sammmeee guitar haha great job!

reinovator

says: Aug 10, 2010. 8:57 PM REPLY

Thank you for keeping PURE amplification Alive.This is a keeper!! Thank you.

rode

says: Aug 8, 2010. 4:58 PM REPLY

Very well explained - maybe too well, as those with no experience might be tempted. Nice job overall though - very thorough.

There are a lot of comments about this amp, so I may have missed it ... Using t he 7V tap off the main HT winding is a VERY BAD idea, because you can very easily exceed the maximum heater/cathode voltage (which will cause catastrophic failure), and at the very minimum you get hum if there is any leakage.

Just upgrade the wallwart used for the 12AX7 so it can handle all the heaters. All the circuitry operates at hazardous voltages (including the output valve heaters !!!). This is a potential killer - please amend the circuit to use the small auxiliary heater transformer for all valves - both for safety and to protect the heater-cathode insulation.

The position of the standby switch means that as you switch it off, a HUGE voltage spike is generated in the choke because you've just interrupted the current flow. The spike can (and has) damaged the insulation in the choke.

This can be fixed by using diodes (you'll have to work out where they go, it's t oo hard to describe in a short message).

Single-ended Class-A output stages always have asymmetrical distortion which many guitarists dislike. A disconnected speaker will usually cause the demise of the output transformer (same mechanism as interrupting the current through the filter choke).

Hope this is helpful.

gmoon

says: Aug 8, 2010. 7:48 PM REPLY

Thanks for your comments...

The filament winding is indeed bad for hum, but not much of an issue with the power tubes.

I'm not too concerned about safety. The PT is fully isolated and the chassis grounded to earth. Any catastrophic short between the PT secondaries should just fry the heaters (and / or blow a fuse). Bad for the tubes, but not much of a safety issue. A separate filament transformer would indeed be a better solution from the hum perspective, though.

The standby switch was never added to the circuit (and indeed is overkill in a smaller amp), but I agree with your assessment.

(much of this amp has been lar gely redesigned / rebuilt in the last two years; maybe someday I'll post an updated schematic...)

Not sure why you added this. I don't disagree (actually I do disagree about Class-A, at least compared to other topologies), but the dangers of running a tube amp without a load are well known...

rode

says: Aug 8, 2010. 10:31 PM REPLY

gmoon,

If you measure the peak and peak+DC between 6DG6GT cathode and heater, you'll find that it is either right at the very limit of the allowable maximum, or may exceed it. This is generally not recommended & that's what I was concerned about.

The safety issue was mentioned because people expect the heater supply to be "safe" low voltage. Your arrangement causes peak voltages of over 200V on the heater supply.

A properly designed push-pull valve amp will have almost perfectly symmetrical clipping. It's not the Class-A part, but the fact that it's single-ended.

Symmetrical clipping is almost impossible other than wi thin a very narrow range.

There are countless ways to make a valve amp safe (well, safe-ish) with no load, but single-ended output stages are close to impossible to protect.

I rarely make comments on the Instructibles site, but this is an area that I know very well indeed, having designed and worked with valve amps for over 40 years. Just trying to help ;-)

Cheers, Rod

(see http://sound.westhost.com/valves/ for the info I have available).

gmoon

says: Aug 9, 2010. 6:39 AM REPLY Great stuff on your link, Rod....

gmoon

says: Aug 9, 2010. 6:19 AM REPLY

Rod, why would there be 200V on the heaters? I'm not using the 50L6 tubes (seriously--I'd l ike to know if that's the case.) Also, you're aware that two Si diodes have been place in series with the AC heater voltage to drop it below 7V? It's mentioned in the text...

We can argue about the different "sound" of tube amp topologies ad infinitum, but i t'spreference , not textbook math or science, You have yours, I have mine. I enjoy both Class A SE (pushed to clipping beyond it's technical Class A definition), AND I enjoy push-pull AB amps too.

For guitar amps, of course. Not for all audio. And I hardly invented the Class A SE guitar amp...

I still don't know why we're discussing "no load" for this amp. It has a l oad ;-) Do we want t o start a separate theory discussion? That would be OK...

Thanks for the link!

rode

says: Aug 12, 2010. 12:19 AM REPLY

gmoon,

The peak 200V is on the heaters because of the bridge rectifier. I won't try to explain it in a post - but you can measure it with an oscilloscope (10:1 probes are mandatory), or it can be simulated.

Once one side of the output of a bridge rectifier is earthed, the AC winding cannot possibly remain at 0V. The diodes only change the voltage to the heaters, not their voltage with respect to chassis.

Preference (and getting the sound you want) is indeed the dominant force with any guitar amp, but I've heard countless musicians complain about "thin and reedy" sound when their amp becomes asymmetrical for whatever reason, and I just thought I'd mention it.

Cheers, Rod

gmoon

says: Aug 12, 2010. 5:35 AM REPLY

I will measure it, Rod.

Are we talking about an "elevated" voltage in relati on to the chassis ground? Because it's possible that I placed the filament winding at the top of the transformer in the graphic simply to make drawing it easier.

Although when I think about it t hat's irrelevant; windings at either end would have to approach peak V...

REA

says: Aug 8, 2010. 9:05 PM REPLY

isnt it the amps (lol pun!) that kill you, not the voltage?

zack247

says: Aug 8, 2010. 8:36 PM REPLY

i think i have a tube like the one on the far left in step 22. never knew how to use it...

mwseniff

says: Aug 8, 2010. 1:57 PM REPLY

Make sure you fix that Ampeg Gemini II (especially if it has the 15" Jensen Concertone Alnico speaker). I have one and it is my favorite of all my tube amps and probably my all time f avorite amp. Gorgeous reverb and great sounding tremolo as well as accordion inputs, These amps have a truly golden sound using 7591 output tubes which are available again from factories in Russia, 7591'a were actually a hifi tube used by Fisher Radio and other fine

manufacturers back in the 60's probably the sweetest sounding audio output t ube ever made. These also used 7199 driver tubes which are also being made again and the new ones sound quite good. I have been a tube repair tech since I started in the electronic business in 1970 and I have repaired and played guitar thru nearly every mass produced guitar amp made before 2000 so I know what I am talking about here. Good pr oject and you did a pretty fine job for a first time build. I usually build amps on old chassis that I scavenge from old tube organs and old t ube PA amps. You can also find old industrial chassis sometimes that work well. I have a set of chassis from the subsystems of a vacuum tube based electron microscope that was made in Germany that will make some very pretty amps, they are all finished in a beautiful gray enamel paint with chrome trim, many were made to slide in as modules to a large rack cabinet. I also have a big stash of 25L6's that I intend to use as output tubes for a guitar amp. I planned on using 5 in parallel with an isolation transformer for the filament supply. If I string the filaments in series 5 of them will need 125 VAC which should work perfect. Thanks for the great instructable.

gmoon

says: Aug 8, 2010. 7:57 PM REPLY

Great comment!

The Gemini II is l argely refurbished! Unfortunately, when I bought the amp years ago (in 1978) the original speaker was missing. I've got an old Hepner alnico speaker installed, but I'm looking for a better replacement...

But I'm also playing a rebuilt Kalamazoo "Bass 30" as a guitar amp, and that also uses 7591 output tubes, and I love it!

Earthscum

says: Aug 8, 2010. 8:20 AM REPLY

The cathode bypass cap isn't labeled because you choose it to decide at what frequency you want it to boost. The cathode cap and resistor form a high pass circuit, dropping all frequencies above a certain point to ground, causing the amplifier to boost those frequencies. Here's a site with good explanations:

http://www.freewebs.com/valvewizard/ This page in particular shows the r esponse of different bypass caps:

http://www.freewebs.com/valvewizard2/OtherStuff.html

gmoon

says: Aug 8, 2010. 7:18 PM REPLY Thanks for your comment. The "valve wizard" site i s already listed on the "links" page.

However, my comment isn't about the missing bypass cap value, rather the fact that it isn't identified as a cathode bypass cap in the datasheet graphic.

jack002

says: Aug 8, 2010. 2:39 PM REPLY

I was wondering, do your volume knobs go up to 11?

ElJefeUno

says: Nov 30, 2008. 6:55 PM REPLY

Anything above 40 volts is generally considered "high voltage", as it's enough volatage to break down your skin's inherent insulation and allow current to flow across your heart high enough to kill you. I've built tube amps, and I recommend to anyone aspiring to build on that they use extreme caution while working with them. Discharge all caps before working on it (as stated above), and take great care when using a multimeter to troubleshoot. I personally use "gator clamps" when checking high voltages, so that I can do it hands off and not ri sk my life.

dickweed101us

says: Aug 8, 2010. 12:06 PM REPLY

I KNOW THIS IS OLD BUT, This is from Wikipedia The International Electrotechnical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage circuits as those with more than 1000 V for alternating current and at least 1500 V for direct current, and distinguish it from low voltage (50–1000 V AC or 120–1500 V DC) and extra low voltage (<50 V AC or <120 V DC) cir cuits. I just dont like when someone goes saying something that they dont know anything about...

sonofkikkoman

says: Apr 13, 2009. 4:33 PM REPLY

but there is a big difference in 40vac and 40vdc

gmoon

says: Nov 30, 2008. 8:47 PM REPLY

Thanks, good comment.

UnseenBucket

says: May 9, 2010. 12:43 PM REPLY

Hey, awesome guide! do you know of any smaller scale projects? thing is, im only 15 and our school has a bad science department so all this is a li ttle confusing to me, and i wanna be an amp design engineer when im older so i thought experience now wouldnt do any harm, so yeah, is there any other projects i could do to get me start that you know of?

gmoon

says: May 9, 2010. 5:52 PM REPLY

Thanks! Search for "valvecaster"--it's a low-voltage preamp, and a great intro to tubes.

You'd need a working amplifier to plug it into, of course...

mattzerah

says: Jun 26, 2008. 4:20 PM REPLY

Yes, you are correct in saying that its a combination of amps and volts, but when it comes down to it, its the amps that actually determine if you die or not.

The reason a AA battery doesn't kill you is your body provides enough resistance to bring the amps down to a safe level. There are also cases (with car ignition) where thousands of volts may not kill you. Thats because the amps that are produced by the ignition are not enough (although it s border line, dont try that at home, people have died from car ignitions, but some people havn't). The reason home voltage kills you is it runs at 10A (well, in australia it does) and the amount of resistance that your body provides to that just is not enough to bring it to safe levels. I'll stop rambling now and read the rest of the story :) Great project!

gmoon

says: Jun 26, 2008. 6:44 PM REPLY

Lets just say it's a diff erent way of saying "be careful."

The reason a AA battery doesn't kill you is your body provides enough resistance to bring the amps down to a safe level.

That's another way of making my point. For a fixed resistance, increasing the voltage drives more amps. Take any 6V motor, and measure the current draw. Now increase the voltage outside it's operating range, say to 12V. The amperage increases and the motor burns out.

Yes, the amps killed the motor. But it was the extra voltage that drove those amps through the motor coils. So you really need to t hink about the whole Ohms Law equation, not just amps and not just resistance...

(Plus the transconductance of human skin is not l inear--skin shows less resistance as voltage rises, not a linear change dictated by "Ohms Law," but an actual reduction in resistance , which implies that voltage is quite important, too.)

smeezekitty

says: Apr 22, 2010. 9:18 PM REPLY

FYI most 6 volt motors work on 12 volts ;)

gmoon

says: Apr 23, 2010. 5:18 AM REPLY

None that I've bricked...

manumanu764

says: Apr 5, 2010. 4:18 PM REPLY no pcb at all right?

gmoon

says: Apr 6, 2010. 5:30 AM REPLY

Yep, just point-to-point wiring.

Except for the power supply, which I built on a breadboard. I used the breadboard so I could elevate the PS above the end bell of the power transformer, which sticks through the chassis. But terminal strips would work just as well as the breadboard.

Iceberg59

says: Mar 3, 2010. 7:24 AM REPLY

I just came into posession of an early '60s-era Magnavox console stereo of the type that was almost universal in suburban households back then. It works perfectly now -- even the tuner and turntable. The power amp has 4 6V6 tubes (2 per channel). I haven't looked very closely at the preamp yet but I rather imagine it's got 12AX7s in there. It has the typical three tone controls, with the single oddity that the power switches through the treble control pot (which wouldn't be too difficult to fix). My question: Do I have something here that could be converted into a viable guitar amp?

'Berg

gmoon

says: Mar 3, 2010. 1:04 PM REPLY

Very probably yes.

You've got a pair of 6V6 push-pull power amps, each with a dedicated phase inverter (push-pull requires both a non-inverted and an inverted signal input.) 6V6 PP amps were usually i n the 10-12 watt range (that's pretty darn loud with a decent speaker.)

The question is whether or not there's a functional preamp stage--sometimes the radio / phonographs had their own preamp sections, which fed into the power amp. Regardless, a preamp stage can be added easily.

You could wire it as a stereo amp, too.

And yeah, for some reason a lot of older amps had the on/off swit ch on the tone knob. You can certainly move that.

The power transformer would have to be re-wired to for a three-prong cord--it's unsafe to use a guitar amp unless it has a correct ground ref erence.

Read my instructable on isolation transformers , which has some pertinent information (99.9% sure you won't need the i so transformer, but you will need to add the grounded cord.)

Iceberg59

says: Mar 3, 2010. 2:31 PM REPLY

Oh it's got an apparently functioning preamp, too. The tubes fire up and appear to be rock stable. It's on an entirely separate chassis from the power amps'. There are a number (I don't remember how many at the moment) of 12AX7-looking tubes on it and on -- if memory properly serves -- yet another chassis. I don't think the tuner has its own on-board preamp; the phonograph obviously does and they are discrete components. What's really nice is that there's a sti ll-legible schematic of the thing glued t o the inside of the bottom of the cabinet right next to the preamp chassis, which should be helpful but it won't have specifications like recommended plate voltages and proper biasing. Then again, I'd suspect (assuming I get this project off the ground) that rebiasing would be desirable considering that stereo component manufacturers don't much care to have their amps breaking out with a case of distortion.

In document Guitar Tube Amp (Page 41-46)