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.
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.
Member
Joined 2009
Paid Member
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.
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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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.
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.
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.
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.
Have you considered a similar concept as this project by Tristan Collins?
Merlin High Gain Preamp + Firefly Output – Tristan Collins
Merlin High Gain Preamp + Firefly Output – Tristan Collins
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
Interestingly, Mr. Collins doesn't actually say that he liked his creation!
(Though he does say he disliked it's predecessor, a Firefly.)
-Gnobuddy
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
N5X to AX84 HO build – Tristan Collins
I've been working on a similar project, also with a cathodyne phase splitter. After poking around the web, a 6SN7 push-pull power output stage gets good mention for tone. V+ around 250V.
For the output transformer, 6k primary resistance was found to be very good sounding, even though on paper something around 20k makes more sense. The Hammond 125A has lots of taps so you can experiment with different impedances.
The 12BH7 is another tube which gets pretty good reviews in low wattage guitar amps.
I haven't come across anything on low wattage amps which indicates pentodes have superior tone. All of these amps are using preamp tubes, not small power tubes. Thus these tubes are really aimed at voltage amplification rather than driving lots of current. Maybe that's why triodes are ok too in low wattage applications.
Using a power soak on your regular amp is not a bad option. The Weber Micro Mass does a good job on my 15W Fender Super Champ XD.
With low wattage you can never get to that point of interaction between guitar and amp which only happens at high volumes.
For the output transformer, 6k primary resistance was found to be very good sounding, even though on paper something around 20k makes more sense. The Hammond 125A has lots of taps so you can experiment with different impedances.
The 12BH7 is another tube which gets pretty good reviews in low wattage guitar amps.
I haven't come across anything on low wattage amps which indicates pentodes have superior tone. All of these amps are using preamp tubes, not small power tubes. Thus these tubes are really aimed at voltage amplification rather than driving lots of current. Maybe that's why triodes are ok too in low wattage applications.
Using a power soak on your regular amp is not a bad option. The Weber Micro Mass does a good job on my 15W Fender Super Champ XD.
With low wattage you can never get to that point of interaction between guitar and amp which only happens at high volumes.
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Wow, someone reads my site!
I ended up having three builds with very similar preamps (basically the High Octane but converted to Merlin's High Gain) but with differing tone stacks or power amp sections - self split, EL84/6V6 and EL34.
Of the builds I'd recommend the Merlin High Gain Preamp + Firefly Output. I think I may have hit the sweet spot with that one, perhaps by accident. Gain down and Master up you get the power amp distortion and it is surprisingly loud. Go the other way and it is, well, as Merlin says, High Gain.
At home I play the firefly and EL84 builds through a Tayden 12" True Brit at living room / neighbour friend levels (terrace house...).
All the bits are available from Barry at AmpMaker + an enclosure from BlueBell Audio.
As I see it you've got roughly two options:
- Reactive load and re-amplifying;
- Pre amp distortion;
I see no way to get cone breakup and distortion without annoying your neighbours. That breakup is however part of the sound of a cracked Marshall through 1 or 2 4x12 cabinets. Those amps push the speakers beyond their linear range. Furthermore the amps themselves distort on multiple levels: power supply sag, OT saturation, preamp, phase inverter, power tubes reaching saturation, blocking distortion. I probably forget some mechanisms.
Those old amps with very moderate gain clean up very easy. Very good amps when you just want to use the volume control of your guitar and maybe a boost pedal to push the amp over the edge distortion wise. Not so good for full on metal though.
I have neighbours and a pair of ears which need to be handled with care. So I've build a reactive load for my bigger amps. I based the load on my 4x12 with greenbacks. Same impedance peak at 110 Hz and the same rising impedance toward the higher frequencies. I don't know how to compensate for back emf produced by the cone movement and haven't investigated the influence on the behaviour of the amp. Nevertheless, many people were utterly impressed when hearing the load and it performed way, way better than the Hotplate and Ultimate Attenuator I had back then. Playing the amp flat-out through it is hard on the amp though, but so is playing flat-out through speakers. Output valves replaced every 6 to 12 months (frequent player/usage).
Later I build a complete pedalboard with some commercial and mostly home build pedals. Totally different sound. Not as lively or organic I'd say, but very portable and versatile.
Now a AC4 with your speaker is never going to sound like a 100 Watt Marshall. It's got its own sound and will sound steller with some overdrive pedal or a vintage fuzz in front of it.
- Reactive load and re-amplifying;
- Pre amp distortion;
I see no way to get cone breakup and distortion without annoying your neighbours. That breakup is however part of the sound of a cracked Marshall through 1 or 2 4x12 cabinets. Those amps push the speakers beyond their linear range. Furthermore the amps themselves distort on multiple levels: power supply sag, OT saturation, preamp, phase inverter, power tubes reaching saturation, blocking distortion. I probably forget some mechanisms.
Those old amps with very moderate gain clean up very easy. Very good amps when you just want to use the volume control of your guitar and maybe a boost pedal to push the amp over the edge distortion wise. Not so good for full on metal though.
I have neighbours and a pair of ears which need to be handled with care. So I've build a reactive load for my bigger amps. I based the load on my 4x12 with greenbacks. Same impedance peak at 110 Hz and the same rising impedance toward the higher frequencies. I don't know how to compensate for back emf produced by the cone movement and haven't investigated the influence on the behaviour of the amp. Nevertheless, many people were utterly impressed when hearing the load and it performed way, way better than the Hotplate and Ultimate Attenuator I had back then. Playing the amp flat-out through it is hard on the amp though, but so is playing flat-out through speakers. Output valves replaced every 6 to 12 months (frequent player/usage).
Later I build a complete pedalboard with some commercial and mostly home build pedals. Totally different sound. Not as lively or organic I'd say, but very portable and versatile.
Now a AC4 with your speaker is never going to sound like a 100 Watt Marshall. It's got its own sound and will sound steller with some overdrive pedal or a vintage fuzz in front of it.
Thinking about it - a cathode biased PP or SE with VVR for the PA might fit the bill?
AmpMaker's N5X is an ideal platform to start from for a PA in that case. My N5X build turned in to the Merlin High Gain preamp through tweaking, but with the Power control on 0 you are definitely getting PA distortion at low-volume levels if that is what you're after. And the difference between EL84 and 6V6 is noticeable. Loads of options.
T
Hi Tristan, thanks for the input. I dig your Firefly amp, I've had my eye on it for a while now. I've been on and off thinking about a project like this for a fews years, I thought there'd be a lot more 1-2W circuits out there but there's not much. I'm starting to think there may be good reason for that
. I actually tried the Ampmaker VVR on my 18 Watt clone a year or two back but wasn't that impressed with it's performance. I think I've tried and failed with power scaling and attenuation at this point; I was looking to go down something similar to your Firefly route.
However, given the advice I've had here, and the fact that you weren't too smitten with the Firefly I'm starting to reconsider. Biting the bullet and accepting a little more preamp gain seems the sensible route. After all, there's no substitute for genuine power and ear crushing volume...
What was it about the Firefly you didn't like?
Matt.
I think many people found fly-powered power amp sections were too limiting, weak and buzzy when overdriven, and too quiet for clean tones.
My best compromise so far is a watt or two in the power amp (enough for good clean tone at living-room volumes), but generate the distortion in the preamp - and use a pentode in the preamp.
One way to look at it: if your preamp chain ends in a pentode, what is the topological difference between it, and a power amp? Say your preamp is triode -> triode -> pentode; how different is that really from a Fender Champ, except micro-powered?
Push-pull requires a little more thought, but is also do-able. Have a phase splitter driving two (small signal) pentodes as usual, with resistive anode loads (rather than an output transformer). Feed those two out-of-phase anode signals into a long-tailed-pair / differential amplifier to combine them together (mathematically identical to what the output transformer does). The output of your long-tailed-pair pseudo-transformer is now equivalent to the output of a push-pull power amp.
The difference between EL84 and 6V6 sound was mentioned; I agree, and I believe part of that is the fact that one is a true pentode with 3 grids, while the other is a beam tetrode.
Beam tetrodes tend to generate a lot more second harmonic distortion than pentodes; pentodes tend to generate a lot more third harmonic distortion than beam tetrodes.
To my ears, beam tetrodes are usually more likely to "sing" or "shimmer", while pentodes are more likely to "growl".
Note that there are small-signal pentodes (like the EF86 or 6AK5), and also small-signal beam tetrodes, though they were rarely advertised as such.
For example, if you take a magnifying lens and look at the "pentode" inside a 6JW8, you find there are only four grid support rods visible: only enough for two grids. The "pentode" is actually a beam tetrode, though the datasheet says "pentode".
I'm assuming the Phillips patent on pentodes had expired by the time these tubes were being manufactured, which allowed the manufacturer to call them "pentodes" even though they were actually something quite different.
-Gnobuddy
Thanks Chief, that's a lot of useful information to ponder...
The 6V6 is the Beam Tetrode, yes? IIRC Morgan Jones expeains the differences, so I'll re-read tomorrow. From memory, the Tetrode added a screen between grid and anode, while the Pentode went one further, but there were copyright issues which caused some valves to be 'mismarketed', shall we say
.As an aside, I used to have a Roost 100W head, which ran on a quartet of KT77's, the KT standing for Kinkless Tetrode, as they went some way to fixing the awkward kink in the grid curves?
From my side of the pond, the 6V6 is a pesky American 'tube', I don't think I've ever owned an amp which uses them. Perhaps now is the time to try... They'd be overpowered for this application, would they not? Datasheets for breakfast tomorrow
.RE your first post, feeding a Pentode Triodes; I'm looking at using the two halves of an ECC83 to drive an EF86. The first gain stage will be standard, cool biased, with a switchable second gain stage with a more focused/filtered topology to push the mids. Perhaps not a million miles from the Marshall Super Lead.
Hopefully I'll have some rudimentary circuits drawn before the weekend is out.
Thanks again,
Matt.
A couple of things:
a/ That schematic is copyright information
b/ It's a great sounding amp
I would take the front end of an '800 (2203) with master vol and marry it to the 1W back end.
cheers
Terry
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