Harley Benton GA15

Ordered the cheap Harley Benton GA15 15W tube guitar amp from thomann last week. Har anyone tried it and maybe done some mods?

Here is a schematic i found:


The headphones are connected to the 8Ω winding of the output transformer and disconnects the speaker when inserted? Is that really good practise? There is also no grid resistor to ground at the input.

Any ideas on this circuit?

Adding DC heater would probably make a good start.

the obvious mods:

- check all caps in audio-path and upgrade if necessary (C2, C5, C8 etc)
- upgrade opamps to 5532
- add bias-setting option to V4 and V5
- upgrade opamp supply to +/- 15 or 18V
- upgrade psu caps

Here's some ideas taken from the similar but smaller Epiphone VJ

Reduce R1 to let some more guitar signal in, 10k to 33k or so. ( I used 22k )
Right side of R1 could do with a 1M to ground, as you pointed out.
C1 could be the same value as C4 2u2 rather than 22u for less "mush" at higher gain.
Bin  R4, bin R5 and install a 1M log pot lug 3 from C2, with wiper to V2B for a gain control
lug 1 connected to ground.
Probably need a 100 k to ground from right of C2 before the new pot.

Upgrade caps as they are all probably "cheap and nasty"

MM.

Listen to it first, then report back. I could speculate, but it's a waste of time to be honest.

Cap brands and opamp types aren't going to dominate the sound here... That's a very simple preamp and poweramp with some buffers for eq. and effects sends inbetween. See if you are happy with the gain range, character of the preamp distortion (prefer it Fender-ish, or less farty and Marshall-ish), and the overall tone balance.

Thanks for your ideas. I will of course try it stock first but i don't have it yet and was curious if someone else had done something with it. I also have som different speakers to try, both external and internal.

The really weak point in this amp is the EQ. If the schemo is exact, HF is only +/- 1.5dB @10kHz!
LF is a more useful +/- 5.5dB @ 100Hz and MF is very wide plateau between 400Hz and 8 kHz.
It doesn't have the nice midrange cut at 400-600 Hz examplified by Fender EQ.
This, combined with the poor loudspeaker quality, are not conductive to an impressive sound.
I would start by replacing the speaker and install a twin-T based mid-cut between the first and second halfe of the ECC83.

Regarding the eq, you are in a not terrible situation where the drive is buffered by an opamp (this is helpful as the plate impedance of the previous 12AX7 stage is quite high). This means you'll be able to mimic a passive Fender or Marshall eq. quite easily.

Regarding the preamp itself (ie. the first two stages), I'd consider if you want to take a Fender-ish route, or something along the lines of a low-ish gain Marshall sort of thing. Basically optimise for clean or crunch or somewhere in-between.

The push-pull output might be better than some of the Chinese single-ended designs, as the (likely mediocre) transformer will not have to suffer as much idle current, and may offer more low-end/less saturation as a result.

As it is, the EQ has an impedance of ca. 5k. Almost 20dB are lost in the mismatch with the "plate impedance".
Changing the EQ to a Fender type would need changing the pots and modifying U1A as a high impedance inverting stage.

abbey road d enfer said:

As it is, the EQ has an impedance of ca. 5k. Almost 20dB are lost in the mismatch with the "plate impedance".

Click to expand...

I was thinking a bit too far ahead maybe: the tone stack could be driven by one of the opamp stages; scrap the current stack, configure U1a as a buffer, insert classic tonestack fed by a dummy load to mimic original design (eg. cathode follower impedance), drive output into following "send" opamp, or scrap send and return section if not needed.

Again though, it might be worth listening to the amp first.

Thanks for your thoughts! I have not really found the time to dig into this yet.. But i have listened to the amp stock and it's crap..sounds boxy and muddy like you have plugged the guitar into a cardboard box. Tried it with a 15" fender cabinet and it sounded a LOT better so the first thing will be to replace the speaker. I have some 10" and 12" guitar speakers to try. But i was impressed by the overall build quality for that price.

Think i will scrap the send/return buffers and eq and add a gain control at the input stage. As "abbey" said, the eq is quite useless.. Anyone have any suggestions for a tone control cirquit to put in between the first tube? I mainly use a fender jazzmaster for a something half crunchy sound. I'm quite new to the messing with guitar amps thing so please be patient.  :

Hi, I had this kind of amp to mod for a guy a few month ago. Sure, I' m not surprised the speaker change makes the most spectacular improuvement... ( I guess that for a given design, cheap electronic components can still "work", while the cheapest speaker still sounds... cheap...)

My 2 cents: looking at the schematic, there is allmost all you need to make a cool clean/crunchy little "all tube" amp in this box.For example:

- V2a > "gain" pot instead of R4-R5 (220k-1M pot) > V2b > passive tone stack (Marshall, vox, fender, baxandall or anyone of your taste) >("vol" pot - or not)> input of R23.
- V2a > V2b > tone stack > "vol" pot (typically 1M after the TS) > input of R23.

That's only ideas thrown as-is...

Like said before, I'll add a 1M resistor to ground at the input ( can be soldered directly on T and S of the jack input), and try swapping caps (quality for quality, and values to play with tone)...

You can google ax84, aikenamps, schematicheaven, projetg5, ampage, hoffmanamps, loads of infos and ideas to pick on these site!

also:
http://www.duncanamps.com/tsc/
a cool toy/tool to play with.

Laurent.

Thanks Laurent! I've been reading some more trying to figure out what to do to this amp.

pyjaman said:

- V2a > V2b > tone stack > "vol" pot (typically 1M after the TS) > input of R23.

Click to expand...

Guess you ment R24?

How does this look? With a baxandall tone stack and the other mods. Anything obvious i've missed?

A Baxendall tone stack is not the best gift you can make to a guitar amp. Go for a Fender tone stack.

> A Baxendall tone stack is not the best gift you can make to a guitar amp. Go for a Fender tone stack.

Well, it is a James (Baxendall's article is the feedback tone control), and there are people who prefer the James to the Fender.

Different boats for different folks.

I do agree that you can't go wrong with AB763 plan. Use the Normal channel, use 50K Audio-taper pot where it has 6800 (mid), omit the 220K mix resistor since you only have one channel. Keep the while PI-output plan as-is. Forget NFB (for now). With 12AX7 PI and EL84 finals, it will be very hot, but that's the modern style. For older style, put a 250K audio pot to ground between the 0.047 and the 0.001 caps. With this at "5" or "6", gain structure will be similar to the proven AB763. Turned down, the preamp will clip so you fuzz without the full 19 Watts output; turned up, it will scream 19 Watts total fuzz with a faint strum.

Thanks! Guess i will try the fender tone stack then..

Edited the post now as i think i got the idea..got the schems mixed up at first.

Something like this:



What about the coupling caps? Originally it's 22n everywhere, probably because it was cheap and easy.. Any specific spots i should try different values?

Fender did not put a coupling cap after the first plate, so you don't have to. Look at the tone network caps: they block DC. Penny-savings like this are essential in "classic" guitar-amp design. (And that 7-resistor mess around U2A in the first plan is a very different school of thought.)

Your 1Meg pot after the tone controls is your classic Volume control.

The added pot just before the PI is "preamp strangle", often marked Master Volume. If turned full-up, it works like Leo meant an amp to work. If turned down, the preamp stages strain and then fail to drive the PI stage and output bottles to their full 19 Watts of goodness, so you can distort without annoying the neighbors. Of course the distortion is little-bottle distortion, quite different from push-pull. Mega-Metal may use both preamp and power amp distortion (or very elaborate cascaded preamps with a mostly-clean power amp).

Great thanks! This will be fun to try. I have quite some different poly caps, mustards and micas in my boxes to use.

Theres also a big hum and buzz prblem with the amp. Is it worth the effort to make DC heaters for all tubes? When i have torn out the opa's there's a 12V regulated DC free from the psu. Hopefully there's enough juice to power the preamp tube filaments at least.

Was this "big hum and buzz prblem" there from the start?
I mean, Dc heater won't magically fix a grounding issue for instance...

And I wouldn't take the 1.6A value of the 12VDC supply on the original schematic as a trustworthy indication of the available power on those rails.
(unless the PSU is on a separate board which would be used on another hybrid amp with sandstate power...)
According to the datasheet, a couple of TL072 can draw up to 10mA. Even if the PSU have some margin, you are still far from the 150mA you would need to power only the first ECC83!

Axel

I have not yet opened the amp so i don't know what grounding issues might be found in there.. But it's buzzing even with volume at zero and then it's increasing with more gain. So there is hum from both the preamp and the power amp.

Opened the amp yesterday and i'm really impressed by the mechanical work and the overall build quality. But the pcb, the soldering, wireing etc is a f*%king nightmare. Overheated and cold joints, flux residue that makes it almost impossible to take measurements, a random ground plane with jumpers all over and three different ground wires from the pcb to chassis etc.

If i had some turret board and chassis mount tube sockets at hand i would rebuild it from scratch but at the moment i don't so i will try to use what's in there.