Sub Zero Saturn 5V - EQ Question

Humboldt

I recently bought a small 5W valve amp from gear4music and, at certain settings, sounds ok. It's identical to the HB Tube5 which gets decent reviews. At £118 delivered it was worth a punt.

However, overall I wasn't particularly impressed with the sound, although it's 50 years since I regularly used (bigger) valve amps. Sounds a bit flat unless I turn the volume way up.

So, I tried using it with my ultra-cheap EQ pedal from Thomann and it transforms the amp into a superb sounding monster. Is there a simple explanation for this? Without using the pedal, is there a way to modify the stock EQ?

Keefy

How have you set the EQ pedal? Pic?

randella

The tone control in the amp will be some sort of subtractive RC filter, pretty basic stuff. If you’re using the EQ pedal to boost frequencies you’re going to be out of luck.

Even if you’re using the pedal to cut, modifying the amp circuit to be more selective in the frequencies it shelves is probably going to be more hassle than it’s worth with the existing PCB.

Cheap as it is, that EQ pedal will be a far more sophisticated circuit than anything in the amp’s tone network. 

I guess it depends how much of a project you fancy

Humboldt

Keefy said:

How have you set the EQ pedal? Pic?

I have tried a few different settings for different guitars. The photo has my settings for single coil pickups but I have to roll off the bass for my Knee Jerk humbucker. It all works great, it's just i don't really understand why. Most impressive is the pedals use as a Boost.

https://i.postimg.cc/NFNS6tCp/EQ-Pic.jpg

Keefy

Humboldt said:

Keefy said:

How have you set the EQ pedal? Pic?

I have tried a few different settings for different guitars. The photo has my settings for single coil pickups but I have to roll off the bass for my Knee Jerk humbucker. It all works great, it's just i don't really understand why. Most impressive is the pedals use as a Boost.

https://i.postimg.cc/NFNS6tCp/EQ-Pic.jpg

You have between 9 and 18dB of difference between the mids and the highs/lows. You're unlikely to achieve that by modding what is likely to be a Champ-type circuit. Stick with the EQ pedal, it's working for you!

randella

Looking at the settings, the low and high is boosted but the overall output level’s low - essentially a mid scoop. It might be taking away some of the midrange ‘honk’ that you can get with 8” drivers in small cabs.

I found a schematic for the HB Tube5. It’s superficially similar to the 5F1 tweed Champ but has a 10uF bypass capacitor added to the cathode bias resistor on the first gain stage. Maybe suggests the designers were trying to increase gain in the higher frequencies to compensate for the small speaker ‘woofiness’, a bit like your EQ settings. 

Just a punt, for what it’s worth!

Humboldt

"Stick with the EQ pedal, it's working for you!"

Yeah, it works great with the pedal. The transformation is so pronounced that I wonder why they don't incorporate a better EQ when they make these little amps. Cost, I guess?

Anyway, thanks for all the replies.

randella

Yep, I would say it's cost. The single biggest limitation with that amp I would guess is the size of the cabinet and speaker. Have you tried it with a 10" or 12" cab? I'd bet it'd make a marked difference. Given that a sheet of decent hardwood ply currently costs more than pure cocaine, that'd soon add to the £120 (guess who's on the DIY at the minute ).

Then I guess you'd want a smidge more power, so changing the output stage to a pair of 6V6s in push-pull would do the trick. Add a reverb and trem and you've got a Princeton. More money.

It might be interesting to make the existing amp into some sort of hybrid - keep as is but add an active EQ. Then you'd need regulators for the +/-5VDC opamp rails etc. which would add a bit, but that stuff's cheap compared with making bigger cabinets and adding larger speakers.

I guess with the proliferation of cheap modellers like the Nux and Yamaha THRs there'd be no market for it.

Humboldt

Initially, I did consider purchasing an additional 10" cab (£79) from gear4music, presumably with the same Chinese Celestion, but the EQ pedal makes such a difference that I can't even fully crank up the amp in the 1W setting without causing matrimonial disharmony. I'm absolutely loving the sound of the amp now - big and full - it's just a faff having to use an extra cable and power supply. I'll have to dust off my ancient Champ and try the EQ with that.

randella

Sounds great, I know what you mean about 1W being enough. I have a Blackstar HT1 which is plenty to piss the neighbours off with!

Any guitar-related marital disharmony in my gaff is less the noise, more the number of similar-looking large black boxes cluttering up the place

I’m tempted by one of these - I don’t need one, but…

Dave_Mc

I'm guessing the mid-scoop on the EQ pedal is counteracting the mid hump you often get in those single tone control valve amps. I guess you could add a preset Fender Black Panel tonestack to accomplish the same thing (or with treble/mid/bass knobs) but that's going to be a ton of work and expense to accomplish more or less the same thing...

Dave_Mc

randella said:
I found a schematic for the HB Tube5.

Do you have a link? I'm sure I've searched for it before and come up with nothing!

randella

Dave_Mc said:

randella said:
I found a schematic for the HB Tube5.

Do you have a link? I'm sure I've searched for it before and come up with nothing!

Not sure if it’s the right one, but here.

https://images.app.goo.gl/6NURsKEwJGwxoh9g7

Alex2678

Edit - misread 

randella

@Dave_Mc - it seems there's a slew of these small Champs with different branding that all use the same board. Interesting vid here comparing two different approaches at either end of the price spectrum:

Dave_Mc

randella said:

Dave_Mc said:

randella said:
I found a schematic for the HB Tube5.

Do you have a link? I'm sure I've searched for it before and come up with nothing!

Not sure if it’s the right one, but here.

https://images.app.goo.gl/6NURsKEwJGwxoh9g7

Thanks Should R14 to ground there not be before the resistor going into the first valve stage? I'm probably wrong but IIRC they always used to say that was one problem with the Epi Valve Junior. If I'm right, that's probably not helping things either and probably is making it sound a bit murky too.

Thanks for the video too, I'll check it when I get a chance

randella

Ah yeah, I hadn't seen that! You're right, you'd expect it to tie the input to ground before the grid stopper (R9). I wonder why they did that? Also the 68K value of the stopper is replicating the low input of the Champ (the high input parallels two 68s for a 34K total). 

randella

The more I look at this schematic, the more questions I have. The grid leak resistor is in an odd place, and they've used the 68K 'low' input grid stopper presumably to attenuate the signal a little. They've upped the value of the first gain stage cathode bias resistor slightly to 2K2 (from the 1K5 in the Champ). That's going to reduce the gain a little. Then they've added that 10uF bypass cap I guess to increase the gain again but only at higher frequencies.

None of this is what you'd call mad high gain stuff, but they've gone to the effort nonetheless of adding a snubber to the second gain stage. I can't imagine they were having massive problems with oscillation? Also that's going to knock a bit of treble off, maybe that was the idea.

I'd like to have been in the room when they were designing this thing, genuinely interested in the thought process.

Dave_Mc

randella said:

Ah yeah, I hadn't seen that! You're right, you'd expect it to tie the input to ground before the grid stopper (R9). I wonder why they did that? Also the 68K value of the stopper is replicating the low input of the Champ (the high input parallels two 68s for a 34K total). 

Ah you're even using the correct terms and everything! Can you tell I don't really know what I'm talking about when I don't use them?

But yeah it definitely is strange. A lot of the cheapo valve amps are kind of held back by weird design- it's understandable when it's a company with more expensive models. but when it's not? Unless it's like you said, where they're trying to prevent oscillation and stuff like that. But it is a bit annoying and does make some of the cheaper amps not really the best plan, since they can only sound so good (without mods) if they have some weird design choices.

I know, for example, I've almost never, ever used the low input when I had the choice. There's a reason why almost everyone uses the high input.

randella

Dave_Mc said:

randella said:

Ah yeah, I hadn't seen that! You're right, you'd expect it to tie the input to ground before the grid stopper (R9). I wonder why they did that? Also the 68K value of the stopper is replicating the low input of the Champ (the high input parallels two 68s for a 34K total). 

Ah you're even using the correct terms and everything! 

Hah oh no - apologies, hoping it doesn't come across as a smart-alec flex! I'm learning - slowly - I just find it really interesting, and valve circuits are the perfect antidote to days of computers. I did have a bit of electronics background (degree) but I never used it and it was back in the late 90s so it kind of evaporated. Gradually getting it back.

I always write these posts in the knowledge that the time-served amp guys on here will put me straight if needs be, I've learned quite a bit that way

This circuit I just find ever so slightly odd - they deliberately pass high frequencies with a bright cap on the volume pot, then immediately after, add that snubber cap in parallel with the plate load on the second stage which is going to take some high frequency back out again. Just wonder about the thinking, there was obviously a reason.

And yeah, I don't think I've used a 'low' input on any amp ever

Humboldt

I now have a problem with the amp and will discuss with the vendor.

Had some buzzing so arranged a replacement.