Belcat H50R (Bassman) tube amp mods - Love your thoughts!

[daxliniere]

Hey guys!
First post here, have lurked for a little while.

I just bought a Belcat/SubZero/Strauss H50R 50W all-tube guitar amp. It's a spitting image of a Fender Bassman (solid state rectifier). It sounds pretty great already and I got a bargain! I'd like to make this amp a bit special/different, so some mods are on the table. The amp will only be used in the studio, so sheer volume is of no interest to me - I'm chasing a spectrum of tones.

I have an acceptable amount of electronics knowledge, though definitely not a designer. From my research online, I've come up with a list and I would love to get the community's thoughts, specifically on the bolded points. Here we go!
(Most of the ideas courtesy of Rat Amps and Rob Robinette)

Andy at Rat Valve Amps has a list of mods he performs on the 20w and it seems like some of those won't need to be done. Main resistors in PSU are already wirewound and PSU caps are already same or higher values than original Bassman.

This is the schematic I've been working from:
https://music-electronics-forum.com/...6&d=1386131333

1) check 6.3v heater supply is not over-voltage

2) Tone stack variable bypass - nice and simple mod from Robin

3) HF Cut control - 250-500kA pot + 4.7nF (200V+) between pins 5 of both 6L6 tubes

4) Upgrade PSU caps - same values, but high quality parts (C25, C37, C39, C40, C26, C27, C28)

5) Upgrade tone caps - Is it only C2, C3, C5, C6, C7, C8 and C10 that need replacing with Orange Drops? And do C3 & C5 really need to be 1000V rated?? (Or is it just that those values are typically 1kV?)

6) Fuses - what values? where?

7) Gain Boost Switch - Switchable bypass cap over R10 (0.68uF 25V+)

8) 3-Way Negative Feedback Switch, adjust R27=56k (Use 20k for "JTM45 sound"). NOTE; the schematic is wrong for SubZero-branded model (R27=56k), which is 8R output, not 16R as per Strauss. Also, could I just make this a 50k pot+20k resistor?

9) Pentode / Triode Half Power Switch - short Screen (pins 4) and Plate (pins 3) on DPDT switch.

10) Can you achieve sag simply by under-rating the PSU caps?

11) Output Power Scaling/Reduction - some kind of high voltage mosfet? Anyone know where to find resources on this? Does this just 'dim' the B+ supply or something??

12) Adjustable bias - replace R14 (33k) with 15k+50k linear trimpot

13) There is an unused triode in one of the tubes. I'd like to make use of this in some way. I can see it's pretty easy to make it into another serial gain stage (anode to 100k to point F, cathode to 1.5k to ground), but I'm not sure 1a) the best place to add it or 1b) if I should just parallel the unused triode with one of the other stages. I suppose tonal choices will come down to my personal preference, but are there any positions where you'd suggest it's not worth trying as it wouldn't make a useful difference? I found that Matchless DC30 (very popular in Nashville) has parallel triodes in the very first stage. Apparently it makes things thicker.
I understand that it might be a good idea for this additional serial stage to have a 22-44M resistor and .047uF 400V+ cap connecting grid to plate to form a local negative feedback loop. (Consider putting on a switch)


Any help from the masters would be GREATLY appreciated! Thank you!

Dax Liniere.

[Mick Bailey]

Here's my thoughts on the points in bold;

4. Higher-quality suggest there's something inferior or deficient with the original caps. Is this down to perception or have you looked at specs or taken measurements that suggests they need replacing? If I'm replacing parts I want to determine what the benefit will be in advance and whether this is worth doing. I think it's a mistake just to replace parts that are perfectly adequate.

5. The same with the tone caps. There's an orange drop madness that I don't subscribe to - a blind belief that every amp made would somehow be better if it had every film cap replaced with orange drops. If you must do this then consider the entire signal path including the coupling caps either side of the PI. The 1000v caps are rated at that because it happens to be a convenient commercial rating for smaller caps ather than one that's necessary in a tone stack. They're really common in power supply applications. Usually pale blue in colour but can vary.

6. As a minimum I like to have a fuse in the HT supply. This is because a common failure mode is an output tube short and this can damage the OT and sometimes the PT, which are particularly vulnerable in lower-priced amps. I would insert an F500mA fuse immedely before the choke and then relocate the B+ feed on the choke side of the fuse.

8. Yes.

10. Are you confusing cap rating with value? Sag comes from a combination of resistance and capacitance, that's why tube rectifiers have more sag - there's an intrinsic resistance that increases the time it takes for the main filter caps to charge. There are ways to do this by including series resistance between the rectifier and 1st supply node. You could additionally reduce the value of the PSU caps to give a more 'vintage' value, but at the expense of more hum.

11. Take a look at the London Power articles on power scaling. In the main the preamp side is kept at the original voltage, but the plate and screen voltages are reduced to the power tubes, In advanced circuits the bias is also adjusted. There are some reasonably good output attenuator circuits that are easier, more reliable and less-invasive.
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13. I've just added another unused triode half to an amp as an additional series gain stage. Both are simultaneously controlled with a 500k+500K dual pot (only because I didn't have a 1M+1M to hand). It works really well and gives a nice progression from clean to extreme and makes the guitar volume control far more useful. You may need to experiment with attenuation between the stages so that when at maximum the amp is still useable, or make the 2nd stage cathode bypass cap switchable to knock down the overall gain. No NFB loop required.

[Pedro Vecino]

That amp looks like a copy of the Fender Blues Junior with 6L6.

[pdf64]

Regarding the 'cut' control, power amp negative feedback will tend to diminish its effect, and it may work at odds to the existing presence control.

'9) Pentode / Triode Half Power Switch - short Screen (pins 4) and Plate (pins 3) on DPDT switch.'
Rather than 'short', the screen grid current limiting resistors R35 / R36 should remain in series with the screen grids terminal 4.
Good luck finding a switch adequately rated for this application.

Regarding the general rationale of all these mods, it would be a good idea to try them out first to make sure you actually want them in the long term, before going to the trouble of implementing each mod; especially with it being a pcb amp, it may not be trivial to do all listed.

[eschertron]

My one-and-a-half cents' worth, based on mods I've tried:

Do the easy first.
Replace R27 (NFB resistor) with a resistor-and-pot circuit that can go from less resistance (more NFB and a colder, jazzier tone) to a much greater resistance (less NFB and a hairy-er, raucous tone). Exact values need to be determined by ear. Try to solder and unsolder the pads as little as possible. Maybe attach leads to the PCB pads and do your testing by connecting to the end of the leads.

A tone stack lift or 'variable lift' can extend the range of overdriven tones. You may need to cut traces for this if the tone controls are PCB mounted, as they look.

Rather than experiment with PSU cap values to get a sense of 'sag', add a large-wattage resistor between the SS rectifier and the PSU nodes. Again, you'll determine value by ear. Looks like you'd have to break a trace here too. I'd be a bit reluctant to do that myself.

[Mick Bailey]

The sag resistor is the easier way to achieve some compression. The problem with sag is that it depends on the power amp drawing enough current for it to be noticeable. There's a trade-off in that a larger resistor will give a more pronounced effect at lower volume, but squeeze the life out of the amp at high volume. You can also get some intrusion of ripple and associated intermodulation if the resistor is too large. Maybe try 150R and I'd use an aluminium-clad 25W component bolted to the chassis.

[daxliniere]

Hey Mick Bailey, pdf64 and eschertron! It looks like the forum doesn't send notifications on reply and I didn't think to check back here. Thank you so much for your responses and explanations, that really clarifies a bunch of question marks I had.
There's good and good news. I did a few of the mods, but the ones I haven't attempted yet are mostly the ones you guys replied to!

Regarding upgrading PSU caps, the amp is manufactured to a price point and that's a great place to save money (if you're a Chinese mass mfg). Seems a no-brainer to replace those since new ones were cheap enough. (Only 6 of them, from memory)
That's good to know about the signal path caps. I didn't end up replacing them and the amp sounds freaking awesome. Everyone who records with it loves it.

Here's a recording clip with a little gain from a Klon clone: https://www.instagram.com/p/B31S1A5A94Q/


The mods I ended up adding were:
*Variable NFB - This is so awesome, though I sometimes think I should have just used a 2 position switch. (Not complaining, though)
*Variable tone-stack bypass - This is great too. Brings out a real midrange growl.
*Increased gain of the Boost switch by decreasing cathode(?) resistor and changing a cap. Not only do I get more gain, but it extends the bandwidth in both directions.
*HF cut - This works at a different frequency than the Presence control (a little higher and I would say slower slope) and is perfect for taking the glassy edge off certain tones. Another brilliant tool for recording.

As for lowering the power output, I ended up building an output attenuator box that I can use with any amp. It has 2 switchable paths, resistive and reactive (literally chopped up a 12" guitar speaker) and a couple different attenuation levels (Ohmite 'tapped' ceramic wirewounds).
I decided modifying this one amp for lower output wasn't really worth the trouble.

Regarding fuses, this is something I would still really like to do. Got the fuse holders, just need to get around to doing it. With all the effects of COVID on the music scene, this amp hasn't had as much use as I would have liked.
I'm not doubting your experience, just making an observation, I have a Vox V125 amp with 4x EL34 and it's HT fuse is 1.6A. I did see another forum post elsewhere mentioning 500mA also, so perhaps EL34s draw more current (800mA vs 500mA) or Vox just got it wrong!


Thanks again everybody and sorry for the super-slow reply!

All the best,
Dax.