6AU6 6AQ5 guitar amp

[Paul1453]

I searched the internet and couldn't find a basic guitar amp for my 2 most plentiful tubes.

So I patched a couple of circuits together and it is working on my breadboard.

I have some high frequency oscillation above 50% volume when I use higher voltage +280Vdc.

If I keep the voltage down at +180 there is no oscillation even at full volume.

This is working, and doesn't sound too bad.

Any suggestions for improvement?

I've got a bunch of 6DJ8s.  Would sticking 1 of those in the middle greatly improve the sound?
I'm trying to save my vintage 12AX7s for bigger better amps.

[kagliostro]

If you want to put on a 6DJ8 on the circuit .....

I'll try to use one triode as CF preceded by the 6AU6 and followed by a ToneStack (FMV) or may be a James

then the remaining triode as gain stage (arranged with an adjustable NFB) and the 6AQ5 Power Tube

6AU6 > 1/2 6DJ8 as CF > Tone Control > 1/2 6DJ8 as gain stage + adjustable NFB > 6AQ5

Something like this (or a DC coupled CF instead of an AC coupled CF)



Franco

[eleventeen]

Agree with Kagli, go look at a Dynaco Mk IV and copy the preamp.


http://www.thehistoryofrecording.com/Manuals/DynaCo/Dynakit_Mark_IV.pdf


Indeed, what I am trying to do and have not gotten around to it, is to buy the "upgrade" preamp board from Triode Electronics for a Mk IV and bolt in into a working push-pull power amp to turn an erstwhile salvage power amp into a usable hifi (or stereo) amp. Proven designs are....well, just that.


Yours may be different w/a SE output section. Obviously you do not need a PI but I am not sure a pentode stage can directly drive a pentode output tube without loading down the first stage. I don't see why not, but that's a limitation on what I know and have experimented with.

[PRR]

> Would sticking 1 of those in the middle

The classic g-amp is triode-vol-triode-powertube.

Pent-vol is already prone to overload. It's a sound. Not an all-purpose sound.

Pent-vol-tri-tri-powtube would be way too easy to overload.

If you have far too many 6AU6, strap them as low-gain triodes. Or parallel 5 or 6 as a "power tube".

[kagliostro]

Oh, PRR remembered me I forgot to say one thing

In my option I was considering to use a low Plate resistor value for the 6AU6 as to tame the gain

or to feed it with a low voltage

............ keep all small signal pentodes at low voltages.

see here http://el34world.com/Forum/index.php?topic=17658.msg214485#msg214485)

Franco

[Paul1453]

Thanks for the feedback guys.

The one pentode driving the 6AQ5 I could find was the 6SJ7 - 6AQ5.

I have made a more standard 12AX7 - 6AQ5 which sounds pretty good.

I was thinking that something like kagliostro suggested, with a TS between the triode stages might sound better.

I'm just experimenting and trying to learn.

Hoping to eventually understand how to put some of my excess TV/Radio tubes to good use.

[kagliostro]

Just for knowledge ....

if you are interested in odd schematics with odd pentode as preamp





As you can see the 6AK5 has a low value plate resistor and it is split as to feed the following stage with the wanted tone in a lower level signal

Franco

[Paul1453]

Thanks K,

The 6AK5 is the other pentode I played with driving a 6AQ5 on my BB before.

It wasn't too bad either.  I've got quite a few of those too, but not as many as 6AU6.

The 6AU6 is still common and cheap, same with the 6AQ5.

I'm thinking maybe the input pentode 6AU6 can kick the guitar signal up enough,
that a lower gain triode like the 6DJ8 can run the Tone Stack final gain stage.

Wondering if a circuit like that can sound good as a guitar amp?

[kagliostro]

Tubenit & Geezer give a try to the 6AK5 and told it is a good tube in guitar amp

the 5879 was better so they used the 5879 but here I've found a lot of 6AK5 on the cheap and only few 5879 (that were much more expensive)

so I think I wont to try it sooner or after

6AU6 is surely a good tube in guitar amp

and our friend TIMBO was engaged with this 1950 Moody amp

http://el34world.com/Forum/index.php?topic=19487.msg202001#msg202001



Franco

[Paul1453]

Wow!  Thank you very much, K!

I would have never thought I could get away with 2 6AU6s driving my 6AQ5!

So the 1st one the voltage is kept quite low.

The 2nd one hits it harder, but is tempered by NFB?

No tone control, but I like simple amps with not too many things to fiddle with. 

Time to modify the BB setup to hear this one. 

The 5879s are rather hard to come by, and as you say much more expensive than either 6AK5 or 6AU6s.

I haven't experimented with the 5879, as I think I only have 4 in stock.  Enough to make 2 HoSo58s if I ever get around to it.

I've got a big bag of 6AU6s though, many of them Mullards.  Probably nearly 50 in stock now.

Messing up a few with my dumb experiments/mistakes won't break my heart.   

[HotBluePlates]

... As you can see the 6AK5 has a low value plate resistor and it is split as to feed the following stage with the wanted tone in a lower level signal ...

Merlin has pointed out that the arrangement of the 6AK5 into the cathode follower is a form of bootstrapping: the input impedance of the cathode follower (very high, several-MΩ) becomes the load for the previous stage.

Because pentode stage gain is a function of its transconductance (due to operating point) and load resistance (in this case, very high), stage gain for the 6AK5 will be very large.

[kagliostro]

That is exact HBP and you are right

I was not considering the bootstrap connection stopping my consideration only to the splitted Plate Resistor that is a way to maintain the point of work of the tube and feed the following stage with a lower signal (respect to the signal present directly on the plate)

Thanks for pointing to it, without what I told will be a possible source of confusion

Franco

[PRR]

Yeah, the bootstrapped pentode is HI-gain. (Also about equivalent to the same pentode/triode as two cascaded grounded-cathode stages.)

For low gain in a pentode, put the screen WAY up (even to 300V) and increase the cathode resistor to get current within reason (so a reasonable plate resistor sits around half-way up B+). Very low G2 voltage (~~40V) lead to high gain.

In a bitsa amp (bits of this and bits of that) where power economy is not a big deal, work the pentode at very high current. Like 15K RP and 10mA current. Gain drops roughly as square-root of current. A 10mA design gives ~~1/3rd the voltage gain of a 1mA design. (And hardly needs a buffer behind it!)

[kagliostro]

Hey PRR that is really interesting

A 10mA design gives ~~1/3rd the voltage gain of a 1mA design. (And hardly needs a buffer behind it!)

you say hardly needs a buffer behind it ......

do you mean before a Tone Control that follows a pentode ??

more, which kind of preamp pentode can be pushed to 10mA (like a 12au7 tube) ??

Thanks

Franco

p.s.: Paul I hope you are not angry for the invasion of your thread, I hope this discussion on pentodes is interesting also for you ...

[Paul1453]

I just put my ramblings out there, and see where they lead.   

Then I try to read, reread, and reread again to digest what has been covered.

I forgot I only have 2 7 pin prototyping sockets.
I need to mount another 7 pin socket on the side of my BB to use 3 7 pin tubes.

Trying to keep up, or figure out what you more knowledgeable people are discussing.   

[kagliostro]

Paul here is the link to a project of 5C1 realized using a 6AU6 build by an italian guy (may be you are interested)


http://www.giannicornara.net/pages/page_guitar5-i.htm





Franco

[HotBluePlates]

A 10mA design gives ~~1/3rd the voltage gain of a 1mA design. (And hardly needs a buffer behind it!)

you say hardly deeds a buffer behind it ......

do you mean before a Tone Control that follows a pentode ??

The internal plate resistance of the pentode will be high. But the output impedance of a tube stage can be approximated fairly close as internal plate resistance in parallel with the plate load resistor (rp || Rp).

15kΩ in parallel with anything will be equal to or less than 15kΩ, which is lower than the typical ~39kΩ output impedance of a typical 12AX7 stage.

Hey PRR that is really interesting

A 10mA design gives ~~1/3rd the voltage gain of a 1mA design. ...

... more, which kind of preamp pentode can be pushed to 10mA (like a 12au7 tube) ??

The thread kicked off with 6AU6 which has a 3.5w plate dissipation rating. So you could run that tube at 350v (on the plate, not just supply) and 10mA plate current. But I figure you'll probably have a little bit lower voltage feeding a preamp stage...

[kagliostro]

Thanks HBP

Franco

[PRR]

6AU6 can easily sit with 300V supply, 10mA in 15K, 150V on plate (2.3W Pdiss), gain around 40 (similar to our triodes). 2nd harmonic distortion will be fairly high at high input level (over 1V). Not sure how this is "better" than 12AX7, except it puts homeless bottles to work.

[kagliostro]

............. it puts homeless bottles to work.

and that can be enough

Thanks PRR

Franco

[shooter]

it puts homeless bottles to work.

your witticism continues to amaze me

[vibrolax]

I made a 2 pentode amp (6AK5 pre into 6AK6 power) as a way to build a guitar amp with a tiny power transformer I bought for $2 at a hamfest.  The preamp circuit is nothing special, just one I recycled from an earlier effort.

http://www.frontiernet.net/~jff/pipsqueak_pentode/pentode.pdf

http://www.frontiernet.net/~jff/pipsqueak_pentode/Pentode_DIY_guitar_amp.html

The discussion about how to use pentodes in preamps in this thread is quite interesting.  I hope I can get some time to explore it further.

[HotBluePlates]

I made a 2 pentode amp (6AK5 pre into 6AK6 power) as a way to build a guitar amp with a tiny power transformer I bought for $2 at a hamfest.  The preamp circuit is nothing special, just one I recycled from an earlier effort. ...

And I'm recycling your preamp again! I'm using the same circuit values for the 6AK5 in the pentode channel of 1/3-watt build I'm working on.

The hope is to hear it first, then possibly tinker plate/screen cathode resistor values with a group of decade resistance boxes I have. We'll see if I get a chance to actually do that...

[Paul1453]

it puts homeless bottles to work.

your witticism continues to amaze me

Agreed!

PRR's ability to simplify electronic concepts into something anyone can understand,
like outhouses, and wells is a rare and valuable quality.
Those analogies hit home and stick with me.   

As far as homeless bottles goes.
I've got a Battalion of homeless veteran Soldiers just waiting to be re-deployed.   

Vibrolax, I tried your little pipsqueak amp on my BB before.
It didn't sound bad to me, just pretty low output power.
I want to try to make a really nice sounding circuit,
with around 5 W output (6AQ5) that I can put some of my homeless bottles back to work in.