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Suggestions for a Medium Gain Guitar Amp Topology...
Suggestions for a Medium Gain Guitar Amp Topology...
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Old 6th February 2018, 02:09 AM   #31
gingertube is offline gingertube  Australia
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A simple Amp from 1940's which you may wish to clone/copy.

Historic Harmonica Amplifier

Currently has a triode strapped 6J7 into a pentode mode 6J7 into a 6V6.
Maximum use is made of the 2nd stage pentode as it allows the clever tilt tone control via feedback around the 6V6.

No reason it could not be made using 12AX7 Triode input (paralle the triodes if you like) to an EF86 to 6V6. Solid State rectifier (or 5Y3) OK.

I used that Amp with my Tele for a while and was most impressed.

A guy her in Oz cloned it exactly (except did not use filed coil speaker) and he was likwise impressed.

Cheers,
Ian
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Old 6th February 2018, 07:46 AM   #32
jcarr is offline jcarr  United States
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Quote:
Originally Posted by tristanc View Post
Wow, someone reads my site!
But of course! Amps that can produce tasty sounds at neighbor-friendly levels are (or should be) a high priority for most musicians who live in crowded cities.

Quote:
I'd recommend the Merlin High Gain Preamp + Firefly Output.
The clarification is most appreciated, thank you.

kr, jonathan
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Old 7th February 2018, 03:05 PM   #33
Tubelab_com is offline Tubelab_com  United States
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Quote:
Maximum use is made of the 2nd stage pentode as it allows the clever tilt tone control via feedback around the 6V6.
Years ago I tried sticking the typical TMB tone stack in the feedback loop (OPT secondary to PI) of a typical Marshall style guitar amp. While it did make for some cool and unique sounds, it also made for a rather loud and uncontrolled power oscillator at some settings. The phase shift of a $16 OPT and the tone stack added up to positive feedback.

I have used plate to plate feedback with just a resistor in HiFi amps, usually plate of output tube(s) to plate of driver tube(s).

I had never thought to try combining these two. There is no reason why this couldn't be done in a pentode preamp section either. I guess some more experiments are in order.
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Old 11th February 2018, 07:17 AM   #34
djgibson51 is offline djgibson51  New Zealand
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[QUOTE=Gnobuddy;5327965 Snip...

In the USA, there are also small-signal (true) pentodes that are unloved and dirt cheap, but which sound great. The little (7-pin) 6AK5 is one example. Nobody ever used one in a "classic" guitar amp, so nobody thinks they are worth anything...

-Gnobuddy[/QUOTE]

Hundred Buck Amp Challenge thread, page 70-posts 691 & 692, page 83-post 824.
Still using it.
(Sorry 'bout the Word doc)

Last edited by djgibson51; 11th February 2018 at 07:25 AM.
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Old 13th February 2018, 06:54 PM   #35
LeftHandFool is offline LeftHandFool  United Kingdom
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Good evening all,

First up, thanks to everyone for your suggestions and help over the past week or so. I'd hoped to have had more time to spend on this but life/work/etc. intervened, as ever.
There's a lot been said that interests me, much of which is not reflected below!

Medium Gain Amp V1.png

Much of the above (especially in the output stage) was pencilled in before I started the thread, I'd consider it a starting point rather than a final destination.
Some of the values are more or less arbitrary, and I've lifted the tone stack and 'Boost' wholesale from Merlin's book. I'm not trying to re-invent the wheel, and I'm certain he understands filters and interstage coupling a good deal better than I do .

The pre-amp stages I've designed from the ground up. Again, they're hardly ground breaking but I think they're in the right ball park.
The ECC81 is more or less centre biased, providing enough gain to drive the EF86 (the other triode will drive the reverb circuit), while the EF86 is fairly warm biased, with slightly higher gain.
I'm still considering adding an extra gain stage (probably the other half of the ECC83 in the reverb recovery circuit) as a switchable gain boost between V1 and V2, but it might be prudent to test it without first.

As always, I appreciate the collective wisdom here and your thoughts and suggestions are welcome, particularly where the coupling is concerned. I'll work through it myself in time, but a (gentle) prod in the right direction would be grand.
As ever, you can also have a good laugh at how off my calculations are...

Regarding the output stage, the EL84 circuit I've included here is in some ways exactly what I was trying to avoid using, although it has the advantage of being convenient.
On gnobuddy's advice, I've been looking at the 6AK5 (seems we call them an EF95 over here). They certainly are cheap as chips, and look like a good candidate - I'll be investing in a few in the near future. The different pin out means I'd need to be sure I like them before committing to using them here, so they could be one to try out down the line, perhaps in a low output push-pull amp?
One thing at a time though...

Matt.
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Last edited by LeftHandFool; 13th February 2018 at 06:57 PM.
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Old 13th February 2018, 07:01 PM   #36
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by gingertube View Post
A simple Amp from 1940's which you may wish to clone/copy.

Historic Harmonica Amplifier
Snip...

Cheers,
Ian
Hi Ian,

Thanks for the link, looks like a very cool amp. The tone control is unusual, isn't it, is the idea of including it in the negative feedback loop to make it more linear?

Matt.
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Old 13th February 2018, 09:05 PM   #37
gingertube is offline gingertube  Australia
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The tone control is via local feedback around the 6V6 output tube BUT not including the output transformer.
It is output tube anode to driver pebtode anode feedback sometimes called "Schade" feedback.
It has a second benefit in that it trades a little output tube gm for reduced rp. That gives some improvement in speaker damping.
Because of the output tube anode to driver anode feedback, you want high impedance looking down into the driver tube anode, so as to not divide the feedback voltage too much, hence a pentode driver rather than a triode driver. This feedback voltage division is not as bad as you might think looking at the value of that feedback resistor - why? because that feedback resistors value is "anti-bootstrapped" by having opposite phase signals at either side of it - that is, signal wise, it appears smaller than its actual value.

Doing it that way allows a one knob "tilt" control to be added into that feedback path. That is: flat at mid setting, treble cut and bass boost as you turn down and treble boost and bass cut as you turn up.
The tone control was nice but I particularly recall the the amp for its attack and "bite".
Very responsive to "digging in" on the guitar.

Cheers,
Ian

Last edited by gingertube; 13th February 2018 at 09:07 PM.
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Old 14th February 2018, 02:55 PM   #38
Tubelab_com is offline Tubelab_com  United States
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Quote:
I've been looking at the 6AK5 (seems we call them an EF95 over here).
I have never tried the 6AK5 for an output tube. It does make a decent gain or preamp stage, but it's probably not a good candidate for the first stage due to microphonics. Some are worse than others in this respect, and the military flavors 5654, 6AK5W and 6096 are usually better, but some of them can be screamers too. A screamer is what I call a tube that is mocrophonic enough to start a feedback without a guitar plugged in! (combo, or head sitting on top of speaker)

Quote:
The tone control is unusual, isn't it, is the idea of including it in the negative feedback loop to make it more linear?
Placing the tone control in the feedback loop makes it work "backwards." A treble cut is now a treble boost since the feedback is reduced at treble frequencies. It can also "tighten up" the sound at the frequencies being cut due to more feedback. I haven't tried this particular implementation yet, but it will be tested soon. See the warning below.

Quote:
output tube anode to driver pebtode anode feedback sometimes called "Schade" feedback......you want high impedance looking down into the driver tube.....a pentode driver rather than a triode driver.
We could deal with a low impedance driver tube by lowering the feedback resistor, but the problem with a triode driver is the fact that it's plate resistance varies with the signal swing. This makes the feedback vary with the signal, causing more nonlinearities than it cures. Applying "Schade" feedback to a triode LTP, especially one with a CCS in it's tail usually works quite well. The unbypassed cathode raises the plate resistance into near pentode territory. I have done this in HiFi amps with extremely good results.

Quote:
having opposite phase signals at either side of it
WARNING: A recent flaming failure in a hastily breadboarded high power HiFi amplifier has reminded me of another previous flaming failure. The "Schade" resistor is connected to the plate of the output tube and thus operates at B+ voltage with swings to at least twice B+ (much more in a guitar amp driven to clipping into a speaker load). Its other end is connected to the driver tube plate which will swing towards ground when the output tube's plate voltage peaks. The resistor should be rated for at least twice B+ voltage which is nearly impossible. The high power HiFi amp I am designing will use 4 X 500 volt resistors in series, but I took a shortcut in the breadboard and only used two. A hasty value swap from the top without removing the PC boards left the resistors too close to the board. The insulation failed causing an arc from the end of the resistor connected to the output tube plate to the ground plating on the PC board. Fed by a bench power supply capable of 1.7 amps at 650 volts continuously, the PC board got traces burned off, the resistor vanished, and the OPT is now a magnet.

An experiment in variable UL that wasn't well thought out used a pot connected across the OPT. It quickly proved that the Alpha pots sold by mouser will blow up if used in this manner.

The circuit shown in the harmonica amplifier runs the pot at B+ voltage. There are some series resistors that will prevent fireballs, but I would limit this type of circuitry to relatively low voltages unless the voltage ratings of all parts are known to be suitable. Many pots carry no voltage rating or are rated for 100 to 250 volts.

A quick look at the Mouser Alpha pot datasheet shows that the linear taper pots are rated for 500 volts, while the log pots are 250 volts. I don't remember which kind I blew up, because I didn't realize there was a difference until I just looked them up.
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Old 14th February 2018, 05:28 PM   #39
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by LeftHandFool View Post
...I've been looking at the 6AK5
...They certainly are cheap as chips
...they could be one to try out down the line, perhaps in a low output push-pull amp?
Matt, to be clear, I was thinking of the 6AK5 as a small signal pentode to be used in a preamp, rather than a power amp.

By using a small-signal pentode as the last stage in a preamp, you can get pentode-flavoured distortion out of the preamp. The 6AK5 is a nice inexpensive alternative compared to the much better known (and much more expensive) EF86. It is quite simple to design a normal single-ended gain stage with a resistor as the anode load, just as you'd do with an EF86.

The 6AK5 is rated for no more than 180V (anode voltage), and the data sheets show only about 12 mA cathode current at the "knee" (zero grid voltage, top left corner of the curves.) This makes it challenging to use as an output device, because you need quite a high impedance output transformer.

If one is determined to use a 6AK5 as an output device, djgibson has shown that it can be done. But there are a number of (to use your words) cheap-as-chips small power pentodes out there with more current capability, so if one is going to buy pentodes for an output stage, I think one might as well make life easier by buying something that works with a more easily achievable output transformer impedance.

-Gnobuddy
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Old 14th February 2018, 06:09 PM   #40
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by Gnobuddy View Post
Matt, to be clear, I was thinking of the 6AK5 as a small signal pentode to be used in a preamp, rather than a power amp.

Snip...

-Gnobuddy
Hi,

I had indeed thought you were recommending it as an power tube. At 0.5W max I did think it was a little on the low side, but I assumed there was something I was missing...
I'll re-evaluate it with preamp duties in mind.

Has 'cheap as chips' not made it across the atlantic? I don't think anyone's said it here either. Not since the 1950's, when chips were actually cheap.
The 'chips' in question are - to the best of my knowledge - potato chips (technically, chipped pototoes). Broadly similar to what you would probably call 'Fries'; not micro chips, poker chips, or the things you call chips, which as we all know, are in fact 'crisps'....

Matt.
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