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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 31st January 2018, 07:26 PM   #1
LeftHandFool is offline LeftHandFool  United Kingdom
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Default Suggestions for a Medium Gain Guitar Amp Topology...

Good evening all,

I'm gearing up for a new project, and having been recently reading Merlin Blencowe's 'Designing Valve Preamps for Guitar and Bass', I've settled on a new guitar amp as a good choice.

I've struggled for many years to get a decent tone at levels that don't upset the neighbours, but unfortunately I much prefer the sound of an overdriven power amp to that of cascading preamp gain, rendering the task less trivial than I'd like!
If I'm taking the trouble to design a new circuit, there seems little point in going down the same route most 'classic' amps follow. Initially (and quite arbitrarily), I'm in the mind to try as many different ideas as possible, whilst still creating a design which will sit nicely - tonally - between a guitar and speaker.
Ideally, I'm trying to get an amp that'll comfortably do 'crunch', and clean up with the volume pot, with extra gain on hand (or perhaps at foot) for the gratuitous twiddly bits.

As a point of reference, my current home use amp is a Vox AC4 clone. Using humbuckers into an inefficient 6" speaker I can get it to break up at acceptable levels, but it's not a particularly pleasant sound.
Using single coils it sounds fine, but refuses to break up. That's life...

Moving on, Mr Blencowes book has some interesting ideas I haven't come across before. I particularly like the idea of using a triode gain stage (or two) to feed a pentode - an ECC83 feeding an EF86, being the most obvious choice.

For the output stage, I'm looking to go <4W. Because I already have some, the first choice is to try the EF86 out, either push pull, or dual single-ended.
In the event of push-pull, I'm leaning towards a cathodyne phase inverter, simply because I haven't tried one before.

I'm hoping you more experienced engineers may have some suggestions - particularly relating to the output stage.
What valves are suited to creating low wattage guitar amps? I'm familiar with most of the standard types, but there's a world of choice out there. Does anything stand out from the pack?

Any other suggestions are also welcome. As a project, it's still at the conceptual stage thus far.
The chassis I'll be using is currently punched for 5 noval sockets, but there's room for at least 2 more. HT+ will be around 320V.

Regards,

Matt.
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Old 31st January 2018, 11:15 PM   #2
Bigun is offline Bigun  Canada
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Suggestions for a Medium Gain Guitar Amp Topology...
perhaps the easiest option is a pedal in front of one of your existing amplifiers - many many choices of simple circuits are available on-line and give you adjustable tone.

other choice is to look at adding to an existing amp a) attenuator to suck up excess energy when you drive the amp hard but want it to stay quiet b) power management of the output stage - reduce the B+ to the output stage so the preamp can overdrive it without going to high volume.

However, without pushing volume through the speaker you won't excite resonances and other behaviours that the speaker cone would normally have when being pushed so the tone ain't gonna be exactly the same.
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Last edited by Bigun; 31st January 2018 at 11:25 PM.
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Old 31st January 2018, 11:27 PM   #3
PRR is offline PRR  United States
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Quote:
Originally Posted by LeftHandFool View Post
.......levels that don't upset the neighbours,... ...., I'm looking to go <4W. ..... HT+ will be around 320V.......
Does not add up.

1 Watt is ample to annoy most neighbors. I could probably annoy mine 300 feet away through the woods, except he's usually hammering on a Hudson. In any more neighborly neighborhood, 1 Watt is un-neighborly.

The proposed 300V is way too much for these levels. Output transformers can not be wound to very high impedance, with wide response and good cost. Off the shelf your choices are super limited.

The proposed EF86 will barely deliver 10mA. At 300V that does look like 3 Watts peak, and in push-pull perhaps several Watts audio. But 300V/10mA is 30K each side or a 60KCT winding. That is not something you can just buy.

I had a third-Watt 6AU6 SE plan. That ran 150V supply, but depended on easy availability of the 15K-25K SE Fender reverb OT. I do not know a guitar-range P-P OT of similar impedance.

Then you are down in the kitchen radio power tubes, 6Y6/25L6 and friends, working 110V and few-K loading. You may have to strangle even further to get neighbor-friendly overload power.

OTOH all the tested preamps and drivers are scaled for 250V-350V supply. A 2-level power chain is awkward. On paper you can re-scale the gain structure to distort about the same on 100V as on 300V. I have not seen this done widely.
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Old 31st January 2018, 11:42 PM   #4
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by Bigun View Post
perhaps the easiest option is a pedal in front of one of your existing amplifiers - many many choices of simple circuits are available on-line and give you adjustable tone.

other choice is to look at adding to an existing amp a) attenuator to suck up excess energy when you drive the amp hard but want it to stay quiet b) power management of the output stage - reduce the B+ to the output stage so the preamp can overdrive it without going to high volume.
Hi,

Thanks.

I should probably have said in my previous post that I've tried everything you've suggested and not found the results I'm looking for. As Mr Hendrix himself noted, 'I know what I want, I just don't know how to go about getting it'...

I've been playing guitar nigh on 30 years, and have owned most every kind of amp availabe, from 12 watt solid state 'practice' amps to 100 watt valve heads through a 4x12" Marshall cab.
Those heady days are behind me, although I keep a Marshall 18 watt clone at my workshop for such times as I feel the need to play LOUD.

To be fair, I'm not actually sure what I'm trying to do is possible. As you probably know, standing in front of a wide open 100 watt head is quite a visceral experiance, one which can't be replictaed.
Even my AC4 moves a significant amount of air at full tilt through a pair of 12" greenbacks.
I recognise that dropping the output to 1 or 2 watts is going to be significantly less 'full on', whilst still sounding reasonably 'loud', but since I play the vast majority of guitar in the house these days, I see no harm in trying to design an amp that meets my needs.

It's specifically the design excersie which interests me. I've been through so many combinations of amps/pedsls/atttenuators/speaker cabs over the years, this is the one thing I have yet to try.
Plus this whole forum is about DIY. I'm ready to get my hands dirty.

Matt.
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Old 31st January 2018, 11:57 PM   #5
LeftHandFool is offline LeftHandFool  United Kingdom
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Originally Posted by PRR View Post
Does not add up. <snip>
Hi,

Thanks for that, food for thought, to be sure. I've just made a post in which I agree with some of your concerns - 0W is probably the only truly neighbour friendly output level .

I've done no maths for this project thus far, and i've a feeling that shows. As you may remember form my other threads, I'm not as au fait with the fundamentals of valve circuits as most here. What is instantly obvious to you comes slowly and with much brow furrowing and head scratching on my part.
It's getting late here in the UK, but if I follow what you're saying what I initially proposed is dead in the water from the start?

Plan B would be to stick with a circa 4W output and try to introduce preamp gain in such a way that soothes my ears.

Thanks for the advice, better to accept you're on a hiding to nothing from the outset than continue to flog a dead horse well after dark.

I shall return to the drawing board and not resurface 'til I've got something sensible to say...

Regards,

Matt.
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Old 1st February 2018, 12:04 AM   #6
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by PRR View Post

I had a third-Watt 6AU6 SE plan. That ran 150V supply, but depended on easy availability of the 15K-25K SE Fender reverb OT. I do not know a guitar-range P-P OT of similar impedance.
Reading your reply again, I've just fitted a reverb circuit to my existing amp using a similar transformer. The primary is 22k, if memory serves, marketed as a replacement for the Fender reverb OT.
I'd had this in mind if I did go SE, so there may be some life in my plan. At about 25 each, it's a reasonably cheap part to experiment with.

Matt.
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Old 1st February 2018, 12:25 AM   #7
jcarr is offline jcarr  United States
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Have you considered a similar concept as this project by Tristan Collins?

Merlin High Gain Preamp + Firefly Output – Tristan Collins
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Old 1st February 2018, 12:29 AM   #8
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by LeftHandFool View Post
I particularly like the idea of using a triode gain stage (or two) to feed a pentode - an ECC83 feeding an EF86, being the most obvious choice.
For the last few years, I have intermittently been thinking and working on very much the same issue. I haven't got a final answer by any means, but it has been interesting at every stage of the process.

In the USA, there are still NOS 9-pin triode/pentode TV tubes that can be bought very cheaply (one buck each on sale!) Two bucks for two triodes and two pentodes....or three triodes and one pentode, if you choose to wire up one pentode in triode-mode, screen grid connected to anode via a suitable resistor to kill parasitic oscillations.

So I built a preamp using two 6JW8 triode/pentode tubes. The mostly-clean channel is one triode into one pentode, followed by a master volume. The "dirt" channel is triode-triode-pentode, but the design hasn't been sorted out yet - it sounds like a fuzz-box at the moment, hardly the sound I was going for.

The power amp is built around a pair of little 6AK6 pentodes for about 2 watts output. A little MOSFET "SourceODyne" does duty as a phase splitter. The power amp runs on about 225V. The output transformer has a 22.5k anode-to-anode primary, and is an "OT5PP" from Musical Power Supplies ( Musical Power Supplies, Inc. - Products ). I see they now also have an even smaller OT2PP, 2-watt output transformer with a 25k primary.

2 watts is infernally loud through a guitar speaker in typical urban living quarters. So I rarely have the chance to experience power amp overdrive. Instead, I've focused on trying to get reasonably good-sounding overdrive out of the pentode in the preamp, with mixed results. (It sounds good at very slight levels of distortion, but gets ratty when pushed harder. It needs more development, clearly.)

You have to get creative to come up with the B+ voltage. I used a Hammond power transformer with a nominally 48V secondary. Half-wave rectified, it produces about 75V DC unloaded. Feed a voltage doubler, and you get 150V DC, perfect for the little pentodes in the 6JW8. Feed a voltage tripler, and you get about 225V DC, perfect for the 6AK6 output stages, and the triodes in the 6JW8. Heater power comes from a thrift-store-refugee Sony 8.2V switching power supply, with a power resistor in series to drop the voltage to 6.3 Vdc for all the heaters.

Since it's not 1950 any longer, there is a completely different option for generating the B+: a DC-DC converter, such as this one: Amazon.com: Qianson High Voltage DC-DC Boost Converter 8V-32V to +-45V-390V Adjustable ZVS Capacitor Charging Power Supply Module: Home Audio & Theater

You may be able to run the heaters as well as the boost converter off a single 12V switching power supply; if you use valves with 6.3V heaters, just use them in pairs in your design, and wire the heaters in series in pairs, so you can run each pair off 12 V DC. (You may be able to tweak the power supply up to 12.6V, and I've also seen 13V power supplies.)

Lately I have been thinking a lot about the approach of just making a guitar preamp that generates the sounds I want, and just feeding that into any sort of clean power amp (which might even be a solid-state one, such as a keyboard amp or acoustic guitar amp). Use pentodes in the preamp if necessary, so it is basically a complete little guitar amp, with only milliwatts of output power.

I don't know if any of this is of any use to you, but I thought I'd throw it out there, just in case!

-Gnobuddy
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Old 1st February 2018, 12:33 AM   #9
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by jcarr View Post
Have you considered a similar concept as this project by Tristan Collins?

Merlin High Gain Preamp + Firefly Output Tristan Collins
Interestingly, Mr. Collins doesn't actually say that he liked his creation!

(Though he does say he disliked it's predecessor, a Firefly.)

-Gnobuddy
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Old 1st February 2018, 01:32 AM   #10
jcarr is offline jcarr  United States
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The living-room amp that Mr. Collins declares his liking for is the N5X to AX84 HO, but it is a 5-watt design.

N5X to AX84 HO build – Tristan Collins
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