Amp with 2 6L6s rated at 12W - can I modify to get ~40?

[leevc5]

The schematic attached is for an amp with 3 12AX7s and 2 6L6s, rated at 12W output. The Fender Bassman with 2 6L6s was rated at 40 and the newer Hot Rod Deville with 2 6L6s is rated at 60W.

Is there something I can do to get higher output out of this amp?

[kagliostro]

To have 40W from a project planned for 12W you require a different Power Transformer + Output Transformer

and, may be, some adjustment on the Power Supply to feed the preamp with the planned voltages

With a 40W arrangement of the Power Tubes I would expect a different tone

Franco

[sluckey]

In addition to K's ideas... Convert to fixed bias. Probably be a good idea to use 500 volt filter caps to handle the increased B+.

[kagliostro]

Theoretically, to have the same response from the amp (tone, headroom etc. etc. ....)

you must use the same plan with the number of Power Tubes Triplicate or Quadruplicate

a Power Transformer with same voltage but rated for a higher current and an Output Transformer rated for the higher wattage

Not a nice plan, don't you think ?

Franco

[leevc5]

Thank you for the most excellent replies and recommendations. A 12W amp it will stay and be plenty loud with that glorious 6L6 tone.
If perchance you might see some useful mods they would be gratefully accepted.

[leevc5]

Could I use EL-34s in this amp?

[kagliostro]

A lot of things can be done

but, different headroom and tone

--

With 807 or 1625 you will be near to the 6L6 tone and have the opportunity to get NOS tubes on the cheap than NOS 6L6

but those tube requires "special" sockets UX5 - UX7

Franco

[tubenit]

How did he come up with 12 watt rating with 6L6's? 

[Tony Bones]

My thoughts too. I looks to me like output should be closer to 20W with 6L6. Probably 12W with 6V6.

[leevc5]

12W is what is stated by the seller of the kit.
Ref: http://www.ebay.com/itm/272723248225

[tubenit]

Look at the tube data sheet with push/pull amps:

http://www.r-type.org/pdfs/6l6.pdf

It would seem that he'd have to have pretty low voltage to the power tubes to get 12 watts?  And if he had a real low voltage, then it would seem to be a challenge to have a good sounding reverb?

IF I am understanding the schematic and PT (?), then 260 volts X solid state rectification  (260 x 1.4) = over 360 volts which I think would give over 12 watts of power?

The lowest estimated wattage I've gotten with cathode biased 6L6/5881 is 17w and usually closer to 23 watts with 330-390 volts on the plates. These are a loud 17-23 watts also.

With respect, Tubenit

[PRR]

I suspect it is 17 or 18 Watts as-is.

As said, you need to replace the expensive parts to get more power.

EL34 need pin 1 connected to pin 8, then they will work for a short while. EL34 need more heater power than 6L6, so the power transformer will run hotter. Too hot? Hard to guess. 6L6 will not mind the p1-p8 strap.

6550, KT88 will also run, again straining the PT's heater winding.

7027 will work but NOT if pin 1 is strapped to pin 8! (Amp blows-up instantly.) I have done an amp with 470K p1 to p8 and it seemed to work short-term low-power just fine with 6L6 6550 EL34 or 7027. I am not sure it would work at high power. Because 7027 is now rare, I would just ignore 7027.

6V6 will work at the lower voltage settings. On the 260VAC tap they may glow dull red and have short life.

> How did he come up with 12 watt rating with 6L6's?

I am assuming 8K or 10K OT impedance. For the 40+ ratings on big 6L6 amps we use 4K loading. Also this thing gets just over 400V of B+, and some of that is lost in cathode bias, so maybe 370V available to the tubes. The big amps run higher than 400V plate-cathode.

There is no sin in using big pistons at lower power than "possible". I had a car with an engine rated 250HP in other years, but mine only claimed 138HP. Severely de-tuned to control smog and fuel consumption. OTOH that down-powered engine ran nearly a quarter million miles without any sign of internal wear.

[PRR]

> a real low voltage, then it would seem to be a challenge to have a good sounding reverb?

You can design for whatever voltage you are given.

Odd thing here: the RT says "3W 12V". I suspect it is a little power transformer! This is likely a lower impedance than what Fender used for the small-tube reverb drivers. OTOH the driver is only a 12AX7. It's clearly "less" than the classic Fenders. But unless "endless surf boing" is in the specs, it may be ample.

[Tony Bones]

leevc5, for $37 you get some populated PCBs. I suppose that's a good price if they work. The transformers are another $70. For that, the PT has 260VAC, 25VAC and a 12.8VAC windings. The 12.8V winding could be used to power the heaters if you put the output tubes in series, but that's not the way they show them wired in the ebay listing, which means you'll have some work to do figuring things out.

I'd say the odds are good that this would not ever end up being a working amp.

For $200 you can get one of these: https://www.monoprice.com/product?p_id=611815 It includes a cabinet and speaker. If it doesn't make noise then you send it back. Keep doing that until you get one that works, then get out your soldering iron and power drill.

[leevc5]

Thank you Tony Bones. The Monoprice amp uses EL84 tubes...I don't like the way they sound...too bright, too Marshally...I like the sound the 6L6 makes. I bought a SE 6L6 amp "kit" like this from these guys and it went together quickly just like they showed and worked first time and still sounds great.

If this one doesn't work I send it back, get a refund, bottom feed parts to put a tweed deluxe together...guessing about 250 bucks if I'm patient.

[kagliostro]

I looked at the layout and .... I must say I don't like

Franco

[leevc5]

Would you expand on that please?

[kagliostro]

I'm not considering the schematic in any way (it is what it is)

Looking to the layout

I don't like the fact that there are long connection between the board and the pot (that implies also the use of a lot of shielded cables) and that the pots are on the same side of the tube sockets

also I don't like the way the wires of the tubes (the black ones that go near the chassis corner) are arranged, but this depends on who assembled the kit

Franco

[kagliostro]

I gived a further look to the assembled layout

and ... may be the pots are mounted on the same side of the tube socket because of the chassis was used

seems that the same board, if mounted in a different chassis, can give better results and the connection wires

could be a lot shorter, so ... 37$ may be worth a try

Franco

[leevc5]

Thank you for taking the time to look into it one and all...and in the end we shall see what happens...for $107 if it works a good deal, huh?

Demo: ! No longer available

[kagliostro]

Previously, there was one other post with a sample, the link that you posted now, seems to have problems and didn't work


the sample seems to be here


https://www.youtube.com/watch?v=enQX1ECac9o

Franco

[leevc5]

It is invisible to me, that is what happened when I tried before...

[kagliostro]

Your link is the same I used, but when I try to use your an ftp appears as part of the link

ftp://www.youtube.com/watch?v=enQX1ECac9o


I can see the video on the link on my post (???)

Franco

[TIMBO]

Hey mate, Had a similar thing here. This amp is still going after 50years.
http://el34world.com/Forum/index.php?topic=18639.msg190688#msg190688

[frankenxtein]

Hey mate, Had a similar thing here. This amp is still going after 50years.
http://el34world.com/Forum/index.php?topic=18639.msg190688#msg190688

..... This is all very interesting thanks for sharing.

[tubeswell]

I believe those are from 'KLD' in China. The Chinese factory owner used to lurk around the Ampage site ripping off ideas. For the amount of information he obtained, he could at least have made a $$ donation to support the forum, but he wasn't interested in contributing anything back to the community. Now he's peddling his junk on e-bay. I hope you get a good discount.

[murrayatuptown]

So many years have passed, and maybe nobody cares about this amp...

but I found a schematic on my PC I had downloaded, and it said MOJO 12HR in the drawing 'boilerplate' at bottom right, but no brand. It had a pair of 6L6 on the schematic.

I somehow connected the dots (not many to follow, obviously) between that schematic and KLD MOJO 12HR. That led me to conclude the schematic was incorrectly labeled MOJO 12HR, because the KLD MOJO 12HR does have PP EL84's and 12W is reasonable for them.

I guess that's just a piece of litter blowing around the internet. I should probably delete it from my own downloads so I don't find it again.