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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Wed 13, 2016 11:00 pm 
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mrriggs wrote:
35Z5 wrote:
Yee Hawww...

I'd be riggin' that that Bogen to run a couple 6GT5, those have same basing as 7868...


Tom
Thank You for the suggestion. I spent the night digging through data sheets and drawing load lines. The 6GT5 looks like a perfect match for this output transformer and power transformer (minus voltage doubler). The fact that they have the same basing as the original tubes is pretty slick too.

I plan to "breadboard" this in the Bogen chassis, then after finalizing the design I'll build a chassis to fit the cab and move everything over.
Just a couple of notes. The 6GT5 and, indeed, most any other sweep tube is not the 'same thing' as the 7868. For one, screen voltage is much lower (common for sweep tubes). Also, it requires significantly more heater power (again, common for sweep tubes). The upshot is a more complicated screen supply and it's unlikely that transformer will power a quad. Of course, using oddball heater voltages precludes that too so a separate heater transformer would be needed.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Wed 13, 2016 11:48 pm 
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I didn't suggest a quad, just a pair... Transformer will be loafin'...

Correct me if I'm wrong but I don't see any reason a resistor and filter cap to ground could not be added at the junction of caps in doubler, will give a half voltage supply for screens... Slap the full 450v on plates...


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 1:43 am 
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35Z5 wrote:
I didn't suggest a quad, just a pair... Transformer will be loafin'...

Correct me if I'm wrong but I don't see any reason a resistor and filter cap to ground could not be added at the junction of caps in doubler, will give a half voltage supply for screens... Slap the full 450v on plates...
You're right that with a doubler you can get half but that's still 225 V on the screens that are typically more like 150 V.

Not saying it couldn't be done. Just pointing out it isn't the same as a 7868.

Btw, the ($22) 6GM5 is a 9 pin 7591, which the 7868 was allegedly a copy of, so if he's not keeping the Bogen chassis that's another option and that would give him the full 100 Watt (with a quad). Seems a shame to leave Watts on the table when not needing buy an OPT would more than pay for the tubes and not needing to buy a PT more than pays for the rest.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 2:33 am 
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Max for 6GT5 screen is 220v, should not present a major challenge...


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 2:46 am 
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35Z5 wrote:
Max for 6GT5 screen is 220v, should not present a major challenge...
Not only that but it's Design Center so I wouldn't worry too much about being 5 V off. Stability might be a problem though.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 3:02 am 
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I've run a 6GT5 with 330v on screen and approx same on plate with no issue(no not a circuit I'll discuss here)...


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 3:25 am 
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35Z5 wrote:
I've run a 6GT5 with 330v on screen and approx same on plate with no issue(no not a circuit I'll discuss here)...
Zowie! 330 V eh? I guess that answers that question. :lol:

With 450 V on the plates that's for 100 Watt using that OPT and I'm curious how you intend two 6GT5s to be able to handle it.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 6:55 am 
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Flipperhome wrote:
...The 6GT5 and, indeed, most any other sweep tube is not the 'same thing' as the 7868. For one, screen voltage is much lower (common for sweep tubes). Also, it requires significantly more heater power (again, common for sweep tubes). The upshot is a more complicated screen supply and it's unlikely that transformer will power a quad. Of course, using oddball heater voltages precludes that too so a separate heater transformer would be needed.


Hmmm... Thanks for the heads up. I hadn't yet considered the heater requirements. The heater load on the Bogen is 4 amps and I'm assuming that is the limit of the transformer. A pair of 6GT5's will draw less heater current than four 7868's but I'm still going to exceed 4 amps if I add a reverb and use 6AV6's in place of 12AX7's.

A pair of 6CW5's may be the best fit after all. They are a good match for the OPT, will free up needed heater current, and provide more than enough output power for my needs. I am a terrible guitar player so there is absolutely no need for "gig" volume levels.

Anyone have an opinion on using a 6J6 for a long tail pair phase inverter? I've read that they can be microphonic in audio applications. Some say they work well if you find a quiet tube although I don't think they were building a combo amp.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 12:30 pm 
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mrriggs wrote:
Flipperhome wrote:
...The 6GT5 and, indeed, most any other sweep tube is not the 'same thing' as the 7868. For one, screen voltage is much lower (common for sweep tubes). Also, it requires significantly more heater power (again, common for sweep tubes). The upshot is a more complicated screen supply and it's unlikely that transformer will power a quad. Of course, using oddball heater voltages precludes that too so a separate heater transformer would be needed.


Hmmm... Thanks for the heads up. I hadn't yet considered the heater requirements. The heater load on the Bogen is 4 amps and I'm assuming that is the limit of the transformer. A pair of 6GT5's will draw less heater current than four 7868's but I'm still going to exceed 4 amps if I add a reverb and use 6AV6's in place of 12AX7's.

A pair of 6CW5's may be the best fit after all. They are a good match for the OPT, will free up needed heater current, and provide more than enough output power for my needs. I am a terrible guitar player so there is absolutely no need for "gig" volume levels.

Anyone have an opinion on using a 6J6 for a long tail pair phase inverter? I've read that they can be microphonic in audio applications. Some say they work well if you find a quiet tube although I don't think they were building a combo amp.
Not as easy as you though, eh? :wink: What's making it more difficult is the constraint of trying to fit it to an existing OPT and PT. But that's what makes it fun. I mean, anybody can just 'buy' the stuff. :mrgreen:

We might be able to do something about those heaters. A 6AX7 is a 12AX7 with 6 volt heaters but it's only 7 bucks and with the 6AV6 costing 3 bucks, 6 for two, you might as well use it (and save the cost of a socket) cutting the heater current in half (same as using the 12AX7).

The classic reverb takes a 12AT7 to drive it but the 6DT8 is the same thing (different pinout) for only 3 bucks.

The 6J6 may be okay for a long tail but it's a 450 mA heater and only mu 38 whereas the 6DT8 is 300 mA and mu 60. The 'extra' cost of $3 vs 1 buck seems well worth it, not to mention $2 doesn't really count in the overall scheme of things.

If you end up needing an 'odd' number of tubes the 6AC10 is a compactron triple mu 62 (close enough to the 12AT7). It's 600 mA heater is the same as two 6DT8s but it's only 5 bucks (vs 6, eh) and you don't have to 'waste' a triode. Frankly, I think I'd find something for the 'spare' to do if you used 2 6DT8s and, in any case, the same tube vs throwing in another type is good practice, but it's a thought. Maybe you'll run out of room and the 6AC10 would 'just fit'.

A 'specialized' tube is the $3 6EZ8 9 pin. A triple mu 57 (like 12AT7) with 450 mA heaters (yea! 150 mA per triode) but there's a catch. Two of the cathodes are connected together. Well, hot dog, that'll work for a long tail. But one side of the heater is also tied to the cathodes. Hmm. Maybe the whole heater supply can ride elevated or maybe not. Anyway, just another thought to ponder.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 3:24 pm 
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Flipperhome wrote:
With 450 V on the plates that's for 100 Watt using that OPT and I'm curious how you intend two 6GT5s to be able to handle it.


What is it with you??? Always nit picking... :wink:


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 14, 2016 10:47 pm 
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35Z5 wrote:
Flipperhome wrote:
With 450 V on the plates that's for 100 Watt using that OPT and I'm curious how you intend two 6GT5s to be able to handle it.


What is it with you??? Always nit picking... :wink:
:lol: :lol:

Darn, I was eagerly awaiting your secret :wink:


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Fri 15, 2016 1:32 am 
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Flipperhome wrote:
Not as easy as you though, eh? :wink: What's making it more difficult is the constraint of trying to fit it to an existing OPT and PT. But that's what makes it fun. I mean, anybody can just 'buy' the stuff. :mrgreen:


I'd be disappointed if it was easy. This is my favorite part of the process. Thank You for all your help so far.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Fri 15, 2016 3:27 pm 
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If low watts is okay, I would suggest looking at the AA4 guitar amp schematics found on the 'net. I have built a couple just to recycle old radio chassis, and you can use the old output transformer as well. I have a huge pile of transformers with dual 120v windings, so the power supply is easy. Not exactly "odd tubes that nobody wants" but used ones can be had for very cheap. A couple 6AV6 or 6SQ7 (or 12v version) would replace the 12AX7 for cheap.

Also, if you need any cheap parts, you're not too far away and I tend to hoard things like that, so let me know. I have boxes of 1/4" jacks, power switches, and fuse holders salvaged from old equipment, you're welcome to some of them. I would have offered speakers but wasn't quick enough.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Sat 16, 2016 5:58 am 
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Flipperhome wrote:
We might be able to do something about those heaters. A 6AX7 is a 12AX7 with 6 volt heaters but it's only 7 bucks and with the 6AV6 costing 3 bucks, 6 for two, you might as well use it (and save the cost of a socket) cutting the heater current in half (same as using the 12AX7).

The classic reverb takes a 12AT7 to drive it but the 6DT8 is the same thing (different pinout) for only 3 bucks.

The 6J6 may be okay for a long tail but it's a 450 mA heater and only mu 38 whereas the 6DT8 is 300 mA and mu 60. The 'extra' cost of $3 vs 1 buck seems well worth it, not to mention $2 doesn't really count in the overall scheme of things.


When searching for 6AX7 tubes I ran across the Russian 6N2P which has similar characteristics to the 12AX7 and the same pinout (9AJ) as the 6DT8 [and many others]. From what I've read, the 6N2P's are nice tubes and only cost $2. It seems they haven't caught on with the tube-rolling crowd because to use it in place of a 12AX7 you have to move around the heater leads on the socket.

With the 6N2P and 6DT8 tubes I can basically copy a standard Fender preamp, tone stack, and reverb. Another 6DT8 for a long tail pair phase inverter and there is more than enough heater current left for the pair of 6GT5's.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Sat 16, 2016 7:01 am 
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mrriggs wrote:
When searching for 6AX7 tubes I ran across the Russian 6N2P which has similar characteristics to the 12AX7 and the same pinout (9AJ) as the 6DT8 [and many others]. From what I've read, the 6N2P's are nice tubes and only cost $2. It seems they haven't caught on with the tube-rolling crowd because to use it in place of a 12AX7 you have to move around the heater leads on the socket.

With the 6N2P and 6DT8 tubes I can basically copy a standard Fender preamp, tone stack, and reverb. Another 6DT8 for a long tail pair phase inverter and there is more than enough heater current left for the pair of 6GT5's.
Yup, that'll work. You'll have to run the 6GT5s with the power supply un doubled for about 25 Watt too because the full 450 V would be right for 100 Watt but the plates can't handle it (voltage AND current)


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Sat 16, 2016 7:48 am 
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classicelectronicsguy wrote:
Also, if you need any cheap parts, you're not too far away and I tend to hoard things like that, so let me know. I have boxes of 1/4" jacks, power switches, and fuse holders salvaged from old equipment, you're welcome to some of them. I would have offered speakers but wasn't quick enough.


Thank You, classicelectronicsguy, that is very generous.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: Apr Thu 21, 2016 4:08 am 
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BakelitePete wrote:
I'm building one with two 6av5's in pp, with a triple triode compactron as preamp, and PI. Still looking for output iron that won't break the bank..


Pete


Pete, look for 6FW5, it has better voltage and wattage ratings than a 6AV5, and directly interchangeable. I use 4 in a Bogen LX60 public-address amplifier, totally reliable.

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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: May Thu 12, 2016 6:32 am 
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I'm still plugging away at this, digging up parts and figuring out the schematic. It's funny how hard it is to come up with any technical data for tube guitar amps. You can find thousands of adjectives to describe how one circuit differs from another but very few hard numbers. For instance, I'm trying to work out a negative feedback loop with a presence control. I can't find a single person who has actually measured the dB cut of a standard Blackface Fender setup. I tried calculating it from the schematic and tube charts and came up with about -7dB, which sounds reasonable.

My Bogen output transformer has a separate "70 volt" secondary. The Bogen used this for the negative feedback and I thought I might as well do the same since it's there. I figure with an 82k feedback resistor and a 1.5k shunt resistor it should put it right about -7dB. The 1.5K resistor is paralleled with a 5k pot and 0.1uF cap for the presence control (a setup used on several Fender amps).

This is what the response is with the presence knob turned all the way down.
Image

And with it all the way up.
Image

Is this the right frequency response for a presence control? Again, it's hard to find any hard numbers to work from.

The Bogen had a very small capacitor across the feedback resistor. To me this makes a lot of sense. Feed back all the inaudible frequencies to dampen parasitic oscillations. There are very few guitar circuits that use this "bypass" capacitor on the feedback. And when it is present it seems like guys just cut it out for fear that it is killing their high end. I played around with different values and found that a 47pF cap accross the 82k resistor would start to roll off right at the edge of the audible range.

This is the response with the bypass cap in place and the presence control all the way down.
Image

And with bypass cap and presence control up.
Image

Is there a reason why the small bypass cap is rarely used in guitar amps? Could it actually cause instability in the circuit?


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: May Fri 13, 2016 1:08 am 
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I did not know that the 12AV6 is 1/2 of a 12AX7. In light of that, I think that it would be cool if somebody were to build a lil' audio amplifier using two 12AV6s in a differential pair and two 50C5s in push-pull. The heater string could go straight to the AC line, but the plate supply should run through an isolation transformer for safety reasons.


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 Post subject: Re: One-Dollar-Tube Guitar Amp
PostPosted: May Fri 13, 2016 3:12 am 
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Alfredo_T wrote:
I did not know that the 12AV6 is 1/2 of a 12AX7. In light of that, I think that it would be cool if somebody were to build a lil' audio amplifier using two 12AV6s in a differential pair and two 50C5s in push-pull. The heater string could go straight to the AC line, but the plate supply should run through an isolation transformer for safety reasons.

Hi Alfredo:
Nowadays with low cost, reliable high voltage AC rated film-caps there's no reason to have to try to have filaments which add up to the mains line-voltage.

For instance, I have two PP amplifiers.
One uses four 5902 subminiature tubes the other uses 6V6 or 6AQ5 tubes.
viewtopic.php?f=12&t=273358

These tube filaments are rated for 6.3v @450ma.

I connect them in series for 25.2vAC @450ma.

I connect them directly to the 120v AC mains in series with a 10uf 180vAC dropper-cap.
( I use a 50k bleeder resistor across the cap to discharge it on power off)

The greatest part of it all is that the dropper-cap is a "watt-less" device and there is no heat dropped across the capacitor. Cold as a brick.

The dropper-cap does a great job of dropping the excess 95v AC mains voltage while delivering ONLY the required 450ma, so it's acting as a current controller.
There is a nice slow gradual warm-up... works very well.

The 10 uf film cap I use costs only a few bucks.
http://www.ebay.com/itm/AC-180V-10uF-5- ... Swys5WVnnJ

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