OTL amps for guitar duty

Status
Not open for further replies.
Would a 7695 cut it for 32 ohm? I got four. Other than that a bunch of 12L6's or 6CW5's are about the only higher current tubes I got. Not really interested in getting rid of the OT as if it was weight that was an addition the two pounds is not a big deal once you get the power transformer in the switching realm. Had a thought, but wouldn't you need to compare the same tubes going through a transformer to really compare the two?
 
Last edited:
The 7695 and related 6 volt flavor the 7754 were designed for audio use, and primarilly for the "600 ohm OTL" circuit on the last page of the data sheet. Being an "audio tube" there is no hard spec for maximum cathode current. I have one of each, so I haven't tried them in anything.

Connecting a speaker directly to a vacuum tube is like trying to drive your car 30 MPH in 5th gear. The engine (tube) needs to be running at enough RPM (voltage) to flow air properly and make enough TORQUE (move enough current) to do it's job. The transmission and differential (OPT) multiplies torque (current) at the expense of RPM (voltage).

We can find a big enough engine (tube) to make the required torque at a low RPM, but it will be operating outside of its design parameters and will dissipate a lot of its energy input as waste heat.

So, lets find our big engine that will provide the lowest losses when operated at a low voltage and high current. Here we want 10 watts into 32 ohms. That's roughly 18 volts RMS or 50 volts peak to peak across the load. Peak current into a resistive load should be about 800 mA, but slamming a cone reversing transient into a speaker (say popping the clutch) will need more current.

We only need 50 volts of signal swing, but most tubes need 50 to 100 volts of headroom to get this. Few tubes can pull their plates below 50 volts at the typical current they see, but now we want a low saturation voltage at nearly an amp of current. I'm not sure what the 7965 will do when asked for this. Some tubes that I have tested will arc from plate to cathode resulting in tube death. Others will simply glow red and not pass the current (not enough cathode emission). Here multiple tubes in parallel are required.

As the OTL people have discovered over the years the big regulator triodes are the best tube for this job. They range from tiny (6S4 and 12B4) small (7233 and Russian 6C19) mid sized (6AS7, 6080, 5998 and 7236) big (6336 and 6528) and HUGE (7241 and 7242). Some of these are extinct and expensive.

When I was searching for the best tube in a cathode follower amp which used a modulated B+ supply (class H) I needed big current at low saturation voltages. Some big TV sweep tubes are better suited for the job than the regulator triodes, but require the added complication of pentode operation. I used a big regulator triode for that amp design for simplicity.

I have over 100 25DN6 tubes that cost me $0.50 each, so I may experiment with them, but look at these curves for a BIGGER sweep tube. Yes, that's over 1 amp of plate current at 50 volts on the plate. Short tests (about 500 mS) to nearly 1 amp were done at 650 volts. Rated CONTINUOUS cathode current is 400 mA and the peak rating is 1.4 amps. That can slap some speakers around.

I am including the magazine article I published about 10 years ago that was the result of all this cathode follower stuff. It was removed from Circuit Cellar's web site long ago.
 

Attachments

  • LW6_tests.pdf
    86.1 KB · Views: 58
  • 2910014_Anderson.pdf
    289.1 KB · Views: 51
I think that a sweep pentode, in pentode operation, is likely the best choice for guitar amp OTL. The high Zout is no problem. It actually helps with big cones.....
The two extremes are Fender and Vox. The former uses pentodes with fbk to get a clean sound and the latter uses pentodes without fbk to get a sound that is never really clean.
I use pentode operation without feedback in all my amps and I am able to get super clean output just staying in class A or very near. Plate efficiency is a bit lower than usual but still better than triodes....up around 40-50% with an output transformer.
 
Last edited:
Hey George, you take it as a challenge and that 's fine.

Someone else is certain but cannot make an example. That's not fine.

Calling you out on that one. My surmise is you didn't look at the link I posted earlier.

I got a pair of power transformers for the output at a local surplus outlet (Acme Electronics, defunct since losing their lease) for $6.00. I can't tell what brand but they are marked 'A10310B' and have 117V in and 92.5Volts out. Later I added a control transformer to buck the AC another 12 volts for a little more power.

The 6080s were obtained at a ham swap meet (Midwinter Madness although it was in March) for $2.00 each. 6080s usually don't hold up as well as 6AS7Gs but for $2.00 each I wasn't complaining. A 6SN7 was sufficient to drive them. Sockets came from a coffee can full of used sockets in my basement. I might have $5.00 into that can; had to clean up the sockets prior to use. I found a filament transformer (Signal Transformer Co.) that did 10amps for $10.00, this time from Ax-Man Surplus. A Bud chassis served; $15.00. I used one of our driver power supply transformers for the 6SN7 B+ and B- (I ran it as a cathode follower). That transformer was $12.00 when new (its been sitting on the shelf here for about 20 years). Two 12AX7s suited as the voltage amplifier and preamp. I suppose I have another $20.00 in junkbox parts to put it together with a switch and fuses, Alps controls and Switchcraft 1/4" phone jack. Filter caps gobbled a bit of the budget but no worries coming in under $100 total let alone 100 Euros.

The circuit runs zero feedback. I'd need another gain stage to add feedback. That's why a higher impedance load works best. Otherwise it tends to be too bassy. A box with several drivers in a line source array works pretty nice as the sound pressure does not drop off so quickly as you move away from it.

In my regular amps (production prototypes with more finished chassis work) I've made two versions (each using four power tubes). One uses a direct coupled cathode follower as the driver similar to the prototype above. The other uses an Edcor transformer to convert from single-ended to balanced output for the grids of the power tubes.

Since I'm not running a whole lot of bias current, I don't have either a bias or DC Offset control since the DC Offset current is minimal if the tubes are OK. Its not class A, but it has no crossover artifact since there is no magnetic field to collapse.
 
1W might be fine

I think that you can get there from here.

The 7695 physically looks like a 6W6 / 12L6 type with added fins on the hot sides of the plate for a bit more dissipation. Typical operation has it idling at 100 mA in SE class A. The heater is also 7.5 watts, so maybe it can do the same thing as a 12L6 which is rated for 60 mA average / 180 mA peak in TV sweep use. Assuming that any of these can do 100 mA idle, 200 mA peak, a pair in parallel can put enough current into 32 ohms for a bit over 1 watt.

1 watt into 32 ohms needs 5.7 volts RMS at 175 mA. Peak currents should be in the 250 mA range and peak to peak voltage swing should be around 16 volts, but that's driving a resistor. A 8 ohm guitar speaker near resonance can be 20 ohms or more, and resonance is often within the guitar's frequency range for smaller speakers. I imagine any B+ voltage that gets the tubes to work will provide plenty of available voltage swing.

The (conservative) 10 watt dissipation rating comes into play as B+ goes over 100 volts on the 6W6 / 12L6 if you want to stay in class A. The 7695 can eat 16 watts.

There are several different circuit topologies that can be used for OTL. The totem pole like SemperFi's design, the Circlotron like Atmasphere uses, the Futterman, and the SRPP like circuit shown in the 7695 data sheet.

It has been postulated that an SE OTL would be the worst of both worlds, but it just might work for a guitar amp, and it offers some other circuit choices like the SRPP with some minor tweaks.

Why would I go there.....because I, and others, have been there....a long time ago. Some of the experiments are on a forgotten page in my web site:

Active Loaded SE Output Stages | Tubelab

and the link leads back to some experiments here:

Active load output stage

and to an amp by Pete Millett that looks like an EL34 version of the SRPP thing on the 7695 data sheet.

EL34 Active-load (SRPP) amp

Granted all of these experiments were done with OPT's or big resistors, but the concept can be applied to OTL. All of the circuits are similar with minor tweaks. I would probably use a zener to control the screen grid voltage in pentode mode.....but maybe not....I'll have to try it.
 
Calling you out on that one. My surmise is you didn't look at the link I posted earlier.
I can't see anything but things related to music.

I got a pair of power transformers for the output at a local surplus outlet (Acme Electronics, defunct since losing their lease) for $6.00. I can't tell what brand but they are marked 'A10310B' and have 117V in and 92.5Volts out. Later I added a control transformer to buck the AC another 12 volts for a little more power. The 6080s were obtained at a ham swap meet (Midwinter Madness although it was in March) for $2.00 each. 6080s usually don't hold up as well as 6AS7Gs but for $2.00 each I wasn't complaining. A 6SN7 was sufficient to drive them. Sockets came from a coffee can full of used sockets in my basement. I might have $5.00 into that can; had to clean up the sockets prior to use. I found a filament transformer (Signal Transformer Co.) that did 10amps for $10.00, this time from Ax-Man Surplus. A Bud chassis served; $15.00. I used one of our driver power supply transformers for the 6SN7 B+ and B- (I ran it as a cathode follower). That transformer was $12.00 when new (its been sitting on the shelf here for about 20 years). Two 12AX7s suited as the voltage amplifier and preamp. I suppose I have another $20.00 in junkbox parts to put it together with a switch and fuses, Alps controls and Switchcraft 1/4" phone jack. Filter caps gobbled a bit of the budget but no worries coming in under $100 total let alone 100 Euros.
If the cost of a 50-100 VA transformer is $6 then my amp is quite cheaper than $100. I have made the PSU myself and thought it would cost $30-35. It's highly interleaved for ultra-low capacitance...not the usual cheap stuff....

To make a 16K transformer you can use the Edcor XPP15-8K with 4 ohm seconday and use it with a SINGLE standard (and good) 8R driver...!!. It costs $22.67. This is rated 15W I suppose at the US line frequency of 60Hz which means about 27W at 80Hz if 8K and over 13W if 16K. So it's good for both the 10W Class A PCL86 amp and the 20W EL84 amp! $30 max for chassis and wooden box and that's pretty much where big money is.....EL84's are cheaper than one might think just selecting a variant which is actually better because of the different pin out which allows higher ratings for plate voltage and g2 voltage to make a very reliable 20W amp. IT also allows to simplify the bias supply too....!! PCL86's like Polamp (old Philips Polish factory) I paid them $0.5 each.


The circuit runs zero feedback. I'd need another gain stage to add feedback. That's why a higher impedance load works best. Otherwise it tends to be too bassy. A box with several drivers in a line source array works pretty nice as the sound pressure does not drop off so quickly as you move away from it.
Already a compromise.....How many drivers??? A 6" cone cannot compete with 10" or 12" cones. With an array of 6" cones it has to be seen, interference will happen for sure, so the sound will be quite dependent on the listening position. This might work under certain conditions (compromise) but it might also not work at all for more than someone.....

In my regular amps (production prototypes with more finished chassis work) I've made two versions (each using four power tubes). One uses a direct coupled cathode follower as the driver similar to the prototype above. The other uses an Edcor transformer to convert from single-ended to balanced output for the grids of the power tubes.
What's the point of all your OTL theories if you use a CHEAP signal transformer anyway???

Since I'm not running a whole lot of bias current, I don't have either a bias or DC Offset control since the DC Offset current is minimal if the tubes are OK. Its not class A, but it has no crossover artifact since there is no magnetic field to collapse.

Still that is another weakness and not really recommendable given what people do with a guitar amp. What happens if the output tube fails?? Because guitar amps are not HiFi amps if they are really used sooner or later output tubes will fail or have to be changed before it's too late.... I have seen enough to be certain that people do not care about checking the status of the output tubes routinely.....
There is no collapse in a properly made output stage with OPT. Also because for best performance the output stage shouldn't clip heavily. Distortion can be better made upstream....
I have actually found a way to wind a relatively inexpensive transformer for unity coupling and thus class B operation just like the McIntosh! For those who can't the twin coupled amp as proposed by Crowhurst is a good solution.
 
Last edited:
Oh I see the picture on Fbk. Sorry I do not have an account and never will.

Anyway if that sort of ship is meant to be a guitar amp you will sell none or near none. The chassis doesn't look light at all. Where is the weight saving? A typical 50W amp with EL34's is a lot more compact and can be possibly lighter than that.
A guitar amp has to be practical. This is a crucial requirement just like the sound! We are not talking about stage amps with multiple arrays as large as a house.
50W on which load? 32R? 64R? more? How do you put the drivers? Are you aware of interference and limited freedom in placing them in order to get best result? If you use 2x16R drivers they cannot be too small and if use 4 putting them into a square or similar is not a good idea. I know this has be done and can still be found but it's not a great idea....
 
I can't see anything but things related to music.
The first link was to my regular website. The second link was to my band.

Atma-Sphere

Already a compromise.....How many drivers??? A 6" cone cannot compete with 10" or 12" cones. With an array of 6" cones it has to be seen, interference will happen for sure, so the sound will be quite dependent on the listening position. This might work under certain conditions (compromise) but it might also not work at all for more than someone.....
If you place them close to each other, and have at least 4, you form a line source array- no cancellation. I used Jensen 8" units and they work quite well.

What's the point of all your OTL theories if you use a CHEAP signal transformer anyway???
To get more of the kinds of distortion made by regular guitar amps without the weight and cost. It works pretty well in that regard- plenty of bandwidth and no worries driving the power tubes into saturation. I run a single-ended front end clear up to the driver tube to get a bit of 2nd-order harmonic richness. Sunn used to make solid state amps that were a similar topology and have a reputation as some of the richer solid state amps made. Their driver transformer wasn't a miracle device either :)


Still that is another weakness and not really recommendable given what people do with a guitar amp. What happens if the output tube fails?? Because guitar amps are not HiFi amps if they are really used sooner or later output tubes will fail or have to be changed before it's too late.... I have seen enough to be certain that people do not care about checking the status of the output tubes routinely.....
There is no collapse in a properly made output stage with OPT. Also because for best performance the output stage shouldn't clip heavily. Distortion can be better made upstream....
If an output tube fails you replace it. Pretty much like other amps in that regard :)


The amp in the photo on our FB page is a prototype. The chassis was designed to be mounted on its side in a combo. But much later on I built it up as a guitar head- we were going to put it upside down in an open back using one or a pair of 10" drivers originally meant for an SVT cabinet.
 
Last edited:
The first link was to my regular website. The second link was to my band.

Atma-Sphere
I knew your website but cannot see any guitar amp there.

If you place them close to each other, and have at least 4, you form a line source array- no cancellation. I used Jensen 8" units and they work quite well.
Rather simplistic assumption. From my past experience you will have interference in high midrange and treble range for sure.
The only full range driver line array that works well is that one using 4" cones MAX size. 5" is already too big! And it has to be rather high to approximate a cylindrical line source. In your 4x8" cones emission lobes will develop.
Moreover its an array so NO way of getting a reasonably sized and/or portable amplifier. Totally unpractical. Only usable if never moves otherwise it's just for museum display.
I am curious to see where do you put the amplifier then. I doubt you can make a reasonable combo amp.

To get more of the kinds of distortion made by regular guitar amps without the weight and cost. It works pretty well in that regard- plenty of bandwidth and no worries driving the power tubes into saturation. I run a single-ended front end clear up to the driver tube to get a bit of 2nd-order harmonic richness. Sunn used to make solid state amps that were a similar topology and have a reputation as some of the richer solid state amps made. Their driver transformer wasn't a miracle device either :)
Without the weight and cost? You cannot make such general statements sorry. You just wrote that for the 50W amp amp you made an array of 4x8" cones. + head. That is heavier than a 50W combo amp with its OTP and a single driver. No question.
You missed my point. You have been raving about the transparent sound of OTL amps and then you use a cheap interstage transformer that is typically worse than an output transformer of the same price range! You confuse transformer saturation and power stage clipping. One doe not imply the other.

If an output tube fails you replace it. Pretty much like other amps in that regard :)
You might need to replace the speakers too....besides tubes in OTL amps, as used in guitar world, have surely shorter life than typical pentodes. So the long term cost quite higher.

The amp in the photo on our FB page is a prototype. The chassis was designed to be mounted on its side in a combo. But much later on I built it up as a guitar head- we were going to put it upside down in an open back using one or a pair of 10" drivers originally meant for an SVT cabinet.

2x10" its not small. For a 50W amp might be fine but a 10-20W amp? Surely not.
If OTL guitar amps had something more to offer or were more convenient be certain that you would not be alone....doing it for fun is one thing but it ends there.
 
Rather simplistic assumption. From my past experience you will have interference in high midrange and treble range for sure.
The only full range driver line array that works well is that one using 4" cones MAX size. 5" is already too big! And it has to be rather high to approximate a cylindrical line source. In your 4x8" cones emission lobes will develop.
Moreover its an array so NO way of getting a reasonably sized and/or portable amplifier. Totally unpractical. Only usable if never moves otherwise it's just for museum display.
I am curious to see where do you put the amplifier then. I doubt you can make a reasonable combo amp.
No assumptions- I've had it working and know what it does for over 18 years. It works fine. I've heard it in a number of clubs in Minneapolis. Its nice because it projects so well and you don't have to go deaf on stage while its doing it.

Without the weight and cost? You cannot make such general statements sorry. You just wrote that for the 50W amp amp you made an array of 4x8" cones. + head. That is heavier than a 50W combo amp with its OTP and a single driver. No question.
Without any knowledge of the actual amp this is a silly comment. It weighs less than 50 watt combos and that was my goal. I shaved off about 15 pounds. When you're hauling that stuff in and out of bars, up and down stairs, that sort of thing makes a difference.


You missed my point. You have been raving about the transparent sound of OTL amps and then you use a cheap interstage transformer that is typically worse than an output transformer of the same price range! You confuse transformer saturation and power stage clipping. One doe not imply the other.
You didn't have a point other than to troll. I didn't miss that.

Clearly as well you have no idea what you're talking about. No experience with the way the transformer was used, no idea of the specs of the transformer... yet you go off as if you know everything about this project and there's no way you can.



You might need to replace the speakers too....besides tubes in OTL amps, as used in guitar world, have surely shorter life than typical pentodes. So the long term cost quite higher.
Triode power tubes typically have a 10,000 hour service life- about 5x longer (we warrant them for a year) than power pentodes. If the amps were damaging speakers I would not be in business right now... Sheesh...


2x10" its not small. For a 50W amp might be fine but a 10-20W amp? Surely not.
If OTL guitar amps had something more to offer or were more convenient be certain that you would not be alone....doing it for fun is one thing but it ends there.
I'm not alone. This thread is years old and I didn't start it. It only came to life recently when you started trolling it.


OTL amps are fun, that's true. And it has ended there so far. I've seen enough OTL guitar amps at shows to know I'm not the only one.

The amp works fine regardless of any of your remonstrations.
 
No you have no idea of what a guitar amp should be

One man's awesome guitar amp is another's box full of noise. There is no one size fits all, although there are some "a few sizes fit a lot" (Fender, Marshall Vox clones).

I have made guitar amps from all sorts of non traditional things from old TV sets to Tiger series solid state amps to traditional FMV clones. I am one to seek out bargains for use in my endeavors and a defunct K-Mart store netted me a trunk load of car stereo speakers some time in the late 90's.

During this time my daughter played in the high school band and her friends spent a lot of time at our house annoying our neighbors with all sorts of musical weaponry. We had a 10 X 18 foot room in the back of the house with a full drum cage, a Roland JV1000, a couple of Strats, a Les Paul, and a cheap bass of some sort. I still have the JV, some of the drums and the Les Paul.

I had built several guitar amps using those car stereo speakers, and within reason they sounded pretty good. They were all SE designs from 4 to 15 watts loosely based on the Fender Champ. Many went to her friends, but there was this Ugly Duckling that nobody wanted. It was electrically similar to the rest, but had nice JBL car speakers that I had removed from my car before trading it in. They were just too clean, and it didn't ROCK, so it wound up doing duty on the JV1000.

One of my daughter's friends gets a new guitar for Christmas and brings it over. He plugs it into Ugly, and magic happened. Ugly didn't rock but she sang sweetly when fed from an ES-335. He wound up taking Ugly home, and eventually off to college, and I never saw her again. There IS a player for every amp....well most every amp.

I have a 100 watt monster in the corner. It is a Chinese clone of a Mesa Triple Rectifier. Twenty Seven kinds of LOUD, but rather lame when turned down to basement friendly volume levels, so it hasn't been turned on in over a year.

I have a little DIY amp that evolved from something I built for the HBAC. It makes about 4 watts when everything is dimed and you wail on it, 2 to 3 watts clean. It cost about $40 at the time of build. The input tube is a bootstrap loaded pentode whose gain can be adjusted from about 20X to over 1000X before microphonics get out of hand. The amp can be adjusted so that saturation comes in the input, tone stack buffer, output, or all 3 stages for a wide variety of tones. This is the only amp I have used for the past 2 years, except for when I just plug my guitar straight into the USB sound module and use an amp sim. I grew up in Florida and learned guitar with 60's surf music, and the sim does a nice job on that slippery reverb sound.

After thinking about this and what Printer 2 said, maybe 10 watts IS too much. a watt or two is probably about right, so lets see what can be squeezed from some small tubes.
 

Attachments

  • Chassis_1_x.jpg
    Chassis_1_x.jpg
    601 KB · Views: 113
  • Chassis_3_x.jpg
    Chassis_3_x.jpg
    520.5 KB · Views: 116
everyone can have his opinion and do what he likes.

And that opinion is likely to change over time. In the 70's I actually owned, and liked a Kustom K-200, and liked it, therefore I traced out its schematic and made myself and some friends a clone or three, then sold the original. It turned out that the circuit came right out of the RCA transistor manual, a fact that Heathkit and others also exploited. The Kustom did have two identical RCA power amps wired in parallel, and therefore made more power than the other two, and would deal with a 4 ohm load. Would I want one today? No, nor would too many others.

if one wants to do it for fun there is nothing wrong with it and also enjoy it.

I am not aware that anyone involved in this thread is actively making guitar amps for commercial sale, and I have probably sold more guitar amps than Ralph has over the years if you count all the stuff I made and sold back when I really didn't know what I was doing. In those days most of my sales were by word of mouth, or rather sound of amp. There was this little school near where I grew up called the University of Miami. The Olson Electronics store where I worked for a couple of years called it the University of Money.
 
After thinking about this and what Printer 2 said, maybe 10 watts IS too much. a watt or two is probably about right, so lets see what can be squeezed from some small tubes.

Depends on ones situation. If you want a squeaky clean with an inefficient 8" speaker then maybe 10W would be enough, or a 100 dB efficient 12" playing distorted may find you with a Loud 1W amp. That is home messing around not in a band mix. I had a guy over at my place interested in buying my G&L Tele and he asked if I had an amp to play through. Said I had a few so I pulled out four, played through a Fender Frontman, a Tweed/Blackface 6K6GT Champ, a 12AB5 Bassman with a 12AU6 first stage. He also brought his Fender Deville as he knew the sound of it. The guy was a decent player, it was no hardship him trying the guitar through each amp. He had fun with each also, each being different.

The 6K6 with an old 10" hifi console stereo speaker was loud enough. He actually played the 12AU5 Bassman quieter but I have a master volume on it. Would really have like him cranking that one with its Fender 12" speaker in it, it is a band friendly amp. This speaker would probably do well enough for me with a 1W amp. Or one of the Yamaha's I have, great sounding speakers but they do like a bigger cabinet.

I did find a 25BQ6 although I don't know the health. I could get a pair for $20, not sure if I am all that interested. I doubt there will be a OMG difference as compared to a OT version. Or within the power range of the OTL, could the transformer version sound as transparent with judicial NFB and run in Class A dissipating a tone of power but biased to only put out a watt or so? Well, maybe a little more interested now.
 
I am not aware that anyone involved in this thread is actively making guitar amps for commercial sale,

I have developed the 10W PCL86 exactly for that. It's not just an experiment. I am not the seller though. In return I will get some PSU and OTP transformers wound (my design) for my personal use.
I think the board with all electronic parts, transformers (and optionally Jensen driver 12" ceramic driver) will be available soon both as a kit and finished product.
The one I have been playing with uses the Eminence with Alnico magnet and has been tuned to be used mainly for jazz. Charming warm sound, rich but not fat. High definition, very good attack without being aggressive.
with the Jensen it is still good amp for jazz. Not refined as with the Eminence but butter with overdrive. Last but not least the Jensen is about 2 dB more efficient which is not a bad thing for 10W amp.
 
Last edited:
I have developed the 10W PCL86 exactly for that.

I don't know the situation in the UK or Europe, but here you market drops to near zero if the amp doesn't use tubes that are available at a guitar / music store. This was the case even in the 90's, hence my "for sale" amp's all used the same tubes from the "list of 10" tubes found ad Sam Ash, the Guitar Center, or other store.

I keep wondering if it makes sense to offer my little 4 tube amp (it used cheap oddball tubes) in kit form for something like $150 US, but have never done it. There are too many cheap Chinese tube amps out there in the under $200 range. Some are not "real tube" amps, just a 12AX7 driving a chip amp, but the average guitar player doesn't know that.

Depends on ones situation.

I don't play too much any more, arthritis and nerve damage has taken much of my ability and my left hand goes numb after 15 minutes or so of playing.

I play the 4 watt amp through a DIY cabinet that I made to use as test speakers. I have a pair and each cabinet has two 6 inch 91 dB "pro audio" drivers. They are not guitar speakers, but can be blasted with 100+ watts without damage. Even with that setup, the little 4 watt amp gets plenty loud when cranked. I can only do that when nobody else is home, and the neighbors hear it even though I'm underground in a basement.

25BQ6 although I don't know the health. I could get a pair for $20, not sure if I am all that interested.

The 25BQ6 is on the dollar list at ESRC. I'm guessing that you could get more than 2 in a box that ships for less than $20 to Canada.

The 6BQ6 tube is where I started in the 60's. They were as common as dirt in the county trash dump, and trust me there was plenty of dirt there too. Wire a plate cap to pin 3 of your tube socket, and they fit into a 6V6 - 6L6....socket. The rest of the pins match. I made several "Champs" in my pre-teen years with 6BQ6's. At the time I didn't understand why the bigger 6DQ6 (same pinout) wasn't louder, and often glowed red, but I was a dumb kid that didn't understand things like impedance, or bias.

There are two kinds of 'BQ6's. The BQ6GT and the BQ6GA. They have the same internals, but the GA version has a bigger plate. Unless space is an issue, you want the GA version.

could the transformer version sound as transparent....maybe a little more interested now.

This thread has awakened my curiosity as well.
 
The 25BQ6 is on the dollar list at ESRC. I'm guessing that you could get more than 2 in a box that ships for less than $20 to Canada.


Dollar list says minimum order of ten. Shipping usually starts around $13-15 US from most US sellers, no idea what they would want for shipping ten but it doesn't hurt to ask. The problem is, if I am ordering might as well what else they have in stock? could easily go down a rabbit hole. Like maybe a 6CD6GA just for fun.
 
I don't know the situation in the UK or Europe, but here you market drops to near zero if the amp doesn't use tubes that are available at a guitar / music store. This was the case even in the 90's, hence my "for sale" amp's all used the same tubes from the "list of 10" tubes found ad Sam Ash, the Guitar Center, or other store.

It's the same generally speaking however this is not a large production and he has a very good stock. He can sell both the amp and the kit with a replacement pair of PCL86's for little more.
He has already made another PCL86 amp, 25-30 amps I believe, about 10 years ago for a guitar manufacturer with the same formula. Apparently no one has complained, yet....
He has sold about 50 HiFi amps both finished and in kit using a single PCL86 SE in triode mode. In that case without replacement tubes.
I am confident it will sell because of the sound. It has been developed especially letting professional guitarists play with it and listening to their feedback. Apart from personal taste there things that everyone wants. Personal taste can be easily satisfied with one or more pedals I think.
So in the end if the amp is really appealing the fact the the power tubes are no more in production, but still available in good numbers, won't stop them from buying, I think. It's not expensive at all looking at what the market offers....
 
Last edited:
Status
Not open for further replies.