HT5 power tube distortion and other concerns

[Laimac]

In the HT-5, the guitar signal goes through the preamp tube for both channels, so a preamp tube change will affect both channels.

Hi guys. Coming in very late on this thread but thought it was the most relevant one to me (thanks a lot for all the helpful advice and remarks) as I am about to change the tube on my HT-5RH. I bought this amp a few months ago as it was highly recommended and I was looking for a cheap TUBE amp to get me going on my second attempt at learning guitar (first one was over 20 years ago... still a bit young for midlife crisis but might be getting there )
I must say I like the amp and its capabilities for the price but I do have a major issue with the way distortion kicks in almost immediately on the OD channel whilst the clean channel cannot be pushed to break-up (and in any case I need to keep things quiet at home) which has gone so far as to discourage me from practicing as much as I should have. I want to play stuff from bluesy to hard rock and this just isn't helpful (yes, I will be using pedals as well - got TC Spark on order and trying to choose).
Anyhow after poring over tons of posts on the topic, I began to understand (I think) that indeed the HT5 only has one preamp tube (Sovtek 12AX7) and the distortion is purely electronic (based on their HT pedal it seems). As a result in order to get a little more usable gain/volume knobs (and get back to practicing), I am about to switch the 12AX7 for a 5751 which has 30% less gain. This will likely reduce the overall volume of the amp (not an issue for me as stated earlier - might even get an attenuator at some point)
It would be great to hear from you guys what you think about the mod, the likely impact on tone and usability (should I have gone for lower gain than 5751?) but most importantly what your experience and workarounds have been so far with the HT5.
Cheers!

[thephantum]

A 5751 will reduce the gain of the amp. There will also be a volume drop, but it's not as noticeable as you think. So from those perspectives, it will do exactly what you want. The problem (at least to my ears) is that, with anything but a 12AX7, the HT5 loses its edge. It becomes flat and lifeless sounding.

Like most tube amps, the HT5 really shines when cranked...especially on the clean channel. It's not until you get the volume up past the halfway point that the amp starts to really show what it can do. So if keeping the dB level low is a priority, get an attenuator. I routinely crank my amps into a Weber Mass III. Not only does it allow me to play at bedroom levels, but it also allows me to dial in my sound at gig/recording levels and then knock off a few dB's to fit into the mix.

My suggestion would be to change both tubes. The Sovtek and TAD it comes with are mediocre at best. At a minimum, swap out the preamp tube from a Tungsol or Miullard. You will notice an immediate difference.

[Adam-T]

This is why I like the HT1R so much , you can thrash it full up without the police arresting you for disturbing the peace, I`m finding that it sounds better than the HT5 too (tried or owned all three HT5 versions - Orig, R and Metal) ..

the HT5 also kinda falls between two stools of too loud flat out at home but not enough (usually) for live . having a 1W and a 30W seems the ideal combination ...

[thephantum]

I get what you're saying about the HT5. I think a lot of what you don't like about it is the stock tubes. Like I said before, get the Sovtek out and put a decent 12AX7 in there....I think you'll be amazed at the difference.

I've posted this in a couple of places (including your other thread)....but, at some point, get a decent NOS 12BH7 in there and have the amp biased warm to hot. The HT5 is designed to run a very warm bias current. I think the main reason for that is because the stock tubes (EH's and now TAD's) are fairly weak sounding versions of the 12BH7. However, from an electrical standpoint it's a tough little tube, so it can take the beating and that makes the amp sound OK with a crappy tube...but get a decent bottle in there and the amp comes to life.

Just as an example, mine came with a EH 12BH7. When I probed that, each triode was running 6 mA bias with 450 V at the plates, which is 77.1% of dissipation (70% is considered warm, 80% is considered hot). I replaced it with a 1950's Tungsol and settled it in at 6.1 mA bias with 453 V at the plates, which is 77.6%. The difference in how the amp sounded was literally night and day.

[Laimac]

Thanks a lot for the quick responses! The main reason why I went for the 5751 is that it is supposed to be closest in specs to the 12AX7 but with lower gain. However I have read that many are changing for 12AY7 or even 12AU7 which I feared might (i) risk damaging the amp (different plate voltage) and (ii) result in a very dull sound (but you seem to say that will be the case with 5751 as well...) On the other hand I am wondering whether the 5751 will give me the gain drop I need. Haha maybe I should have bought another amp after all.
Anyway, came home yesterday to try out my TC Spark and found out that the output section of the amp has broken down (preamp still fine, getting everything through the emulated out) so had to take it to the shop - diagnosis due tomorrow. Talk about a bummer! Hopefully that is just the power tube (which I would then replace with RCA NOS I think) although the tube was still lighting up (I doubt they do that if broken). Amp is still under/near warranty so I did not open it to avoid any claim I messed it up myself.
Next step in my quest for tone (after which I will have no excuse but to learn to play better lol) will be to swap the Celestion 70/80s in my cab for something a little more vintage. Unfortunately speakers are hard to come by where I live, just like tubes which I had to order from the US...

[Adam-T]

Thanks - at the moment, the amp (HT5H non Reverb head) sounds thin, glassy and brash even through a cab with a V30 in whereas the tiny HT1 (which is also running stock valves) does not -

The humble and painfully underrated Laney CUB10 utterly wastes my HT5 in every way regardless of cab connected soundwise though that IS 6V6 based .. Problem with the Laney is that you need pedals for any kind of serious drive, there`s no FX loop and "EQ" is basically a useless treble cut control. it`s a tweaked Pro JR copy with a better choice of power valves .

[Sonofarich]

FWIW...an EH 12AY7 worked well with a Mullard 12AX7 in a HT-20. Better gain sweep and not too sterile. Not sure if it would be the same in an HT5 though.

[Adam-T]

did you have to re-bias for the mullards in the HT20 - It`s the only thing putting me off doing the HT5`s "power" stage (HT1 is fine, it`s all cathode biased) - the last time I biased valves was in the 80s on an old Sound-City 120 head , it was a nightmare swapping resistors and checking with a scope ..Its probably easier with Pots in these modern amps but ......

[thephantum]

Most preamps (including Blackstars) are Class A and thus do not need to be biased. So preamp tubes are literally plug and play. There's a big difference between the HT5 and the HT 20 in terms of the preamp though. Specifically, the HT20 has a preamp tube for the clean channel and another for the OD channel. The HT5 uses a single tube for both channels. The HT20 also reacts better to lower gain preamp tubes than the HT5 does (although to my ears it still sounds like it loses some edge).

Contrary to popular belief, the power amps are very similar. The HT5 just uses both triodes of a 12BH7 in push pull config where the HT20 uses two EL34's in a push pull config. Since they use different tubes they both have different bias points, but in terms of how the amps operate (Class AB), they are very similar

The only time you need to change resistors is on cathode biased amps. Those use a fixed value resistor to regulate bias current. People call them self biasing, but that's a misnomer as you need a soldering iron to set proper bias. Conversely, since it has variable resistor pots, you can bias the HT5 like you would any other fixed bias amp. If you know what you are doing, all you really need is a multi meter and a two screwdrivers (one to remove the chassis and a smaller one to adjust the bias & balance).

Personally, when I bias an amp, I use probes and four meters for each push pull pair. That way, I can see plate voltages and bias currents for both tubes at the same time and dial in an amp (or at least that pair of tubes) in about 3 minutes. It actually takes longer to set the rig up than it does to actually bias an amp, but it still takes less time than doing it with a single meter.

[Adam-T]

I managed to get a NOS Mazda 12AX7 for the HT5`s preamp in the end (the guy mentioned previously had one lurking in a wrong box) and found me a Brimar 12AU7 for the HT1`s output as well .. Both are UK made ...........

if the worst comes to worst and I have to dump the HT5H (if I Can`t sweeten the sound) , I can re-use the 12AX7 in the preamp of the HT1 and be running british NOS valves all the way .. then buy a different head (prob go HT20 or more likely a Laney)