Replacement Valves For HRD IV?

stratman3142

My HRD IV sounds distorted and has a low output, even when I plugged directly into the power amp input.

Am I correct in assuming it's probably the power amp valves?

What replacement set of valves is a good choice and where's a good place to get them?

ICBM

Yes, that sounds like a half-waveform problem caused by half the push-pull output going down, which will give about a quarter of the output power and a permanently distorted sound regardless of volume. If so it *may* be a power valve, or possibly the phase inverter, but it could also be an amp fault - there are a couple of known issues which can do this on Hotrods, although they're not common on the recent versions.

The first thing to do is check that both power valves and both halves of the phase inverter are glowing normally - in the power valves there should be an orange glow near the base and at the top, and usually a slight blue-ish glow on the inside of the glass, most easily visible when you switch from standby to on, and possibly in a darkened room. In the phase inverter there should be *two* spots of orange glow at both the top and bottom of the valve. You may need to take the back panel off to see them clearly, in which case obviously be careful not to put your fingers (or anything else) inside the chassis while the amp is running - take out the six upper screws in the panel, but not the lower ones which hold the valve guard. You may have to slightly loosen the screws on the outside ends of the cabinet to get it to come out easily. (Don't forget to tighten them back up after you've finished.)

stratman3142

There's an orange glow at the top and bottom of the power amp 6L6's, and a blue'ish glow. I can also see an orange glow in the three 12AX7's.

https://i.imgur.com/41QkFJG.jpg

ICBM

stratman3142 said:

There's an orange glow at the top and bottom of the power amp 6L6's, and a blue'ish glow. I can also see an orange glow in the three 12AX7's.

https://i.imgur.com/41QkFJG.jpg

Those look fine, so it’s unlikely to be either a power valve problem or the (relatively) common screen resistor failure - that stops the blue glow in the affected valve.

The next thing to do is swap the phase inverter (nearest the power valves) with the middle preamp valve, and try the power amp input again. If that fixes it, it’s the preamp valve. If it doesn’t, chances are it’s the PI plate load resistor fault which used to be common on them, but I thought they’d fixed now…

stratman3142

Excellent. The power amp input sounds way better when I swap the the phase inverter for one of the pre-amp valves. So that could be the solution.

So I just need to order a 12AX7 for now. Is there a preferred supplier or version of that valve?

PM me, to let me know your consultancy fee (seriously). I really appreciate your help.

ICBM

stratman3142 said:

So I just need to order a 12AX7 for now. Is there a preferred supplier or version of that valve?

I used to use Karltone, but they seem to be closed at the moment - the website indicates they may re-open, but no hint when. It's a long time since I bought new valves from anyone else...

Basically any supplier of modern 12AX7s will do. I would use a JJ (the most reliable) or any Chinese-made one - even if you can find Russian valves now, I don't find them either as reliable or as good-sounding.

Littlejonny

ICBM said:

stratman3142 said:

So I just need to order a 12AX7 for now. Is there a preferred supplier or version of that valve?

I used to use Karltone, but they seem to be closed at the moment - the website indicates they may re-open, but no hint when. It's a long time since I bought new valves from anyone else...

Basically any supplier of modern 12AX7s will do. I would use a JJ (the most reliable) or any Chinese-made one - even if you can find Russian valves now, I don't find them either as reliable or as good-sounding.

I don’t want to go against ICBM as he is the absolute master of amp tech…but, I swapped my JJ 12AX7s for Harma and EHXs. I tried each position at a time and each one improved the sound over the JJs. I can’t remember exactly how but I remember being surprised that valves do actually make a difference. In particular, the drive (and more drive to some extent) is actually now more than useable. I’d go so far as to say with a little external EQ it actually sounds bloody good.

Littlejonny

Edit: I’ve realised that the thread is about a Hot Rod IV…mines a II. Not sure if that makes a difference or not.

p90fool

Littlejonny said:

Edit: I’ve realised that the thread is about a Hot Rod IV…mines a II. Not sure if that makes a difference or not.

Not vast tbh, I have both. The IV has some reliability improvements and a lighter pine cab, but swapping speaker leads over between the two shows that the tonal differences follow the cab/speaker - the amps themselves are practically indistinguishable from each other. 

ICBM

Littlejonny said:

I don’t want to go against ICBM as he is the absolute master of amp tech…but, I swapped my JJ 12AX7s for Harma and EHXs. I tried each position at a time and each one improved the sound over the JJs. I can’t remember exactly how but I remember being surprised that valves do actually make a difference. In particular, the drive (and more drive to some extent) is actually now more than useable. I’d go so far as to say with a little external EQ it actually sounds bloody good.

I realise I was a bit too generalised with that! I actually agree with you about JJs - which are reliable (apart from as cathode-followers, which this amp doesn’t have) but I don’t like the sound of them, although I don’t find it makes much difference as a phase inverter… I wouldn’t use one in V1 or V2. I don’t like the sound of Sovteks much either, but EHs are nice-sounding - although less reliable, and Tung-Sols even more so on both counts. None of the Russian valves are reliable as cathode-followers.

Shuguangs are a good combination of good-sounding and are among the most reliable of the modern valves, and are fine as cathode-followers too, but they’re out of production now. There’s a new Chinese factory, but I haven’t tried any of the valves - it may be a long time until I do, since I’m not doing repairs now and I have plenty of valves…

I’m not the master amp tech either, I just have a knack for remembering all this .

roberty

ICBM said:

I’m not the master amp tech either, I just have a knack for remembering all this .

So an expert, in other words? ;-p

stratman3142

I was a bit unsure about which type of 12AX7 when looking at the various suppliers. So to avoid any ambiguity, I've ordered a JJ 12AX7 from Ampvalves, because I could enter my amp type into their selector, which gave me the recommended valves for my HRD.

There are two types of 12AX7 that come up, and Ampvalves lists a 'balanced' type for the phase inverter. Is the choice of type important?

ICBM

stratman3142 said:

There are two types of 12AX7 that come up, and Ampvalves lists a 'balanced' type for the phase inverter. Is the choice of type important?

No. I wouldn't pay extra for a balanced one, even for a phase inverter - it's unnecessary hype, the phase inverter circuit itself is not balanced so it's not important whether the valve is. If it's no extra you might as well, since balanced valves tend to be closer to the middle of the spec range.

PAL

For tube check out Mike Hill Services . 

stratman3142

Fitted my new phase inverter valve today and my HRD IV sounds like it used to

Top marks to @ICBM.

Out of curiosity, while I'd got the back off the amp, I checked the bias voltage, which was ~75mV. Does that sound about right? There's conflicting info, when I Google it.

ICBM

stratman3142 said:

Out of curiosity, while I'd got the back off the amp, I checked the bias voltage, which was ~75mV. Does that sound about right? There's conflicting info, when I Google it.

There's a major trap with these - if you meter from the test point to the chassis, there's enough resistance between the chassis and the electrical ground to cause a substantial error in the bias reading - always too high, since the true resistance is much more than 1 ohm. As a result, many HRDs get biased far too cold because people turn the bias down to get the 'right' reading. What to do is measure the voltage across the resistor itself, from the test point to the solder pad directly behind it, on the trace that loops around the front of the board - you should find a much more sensible reading.

I also should have said earlier - if the amp is still factory-set for 230V, it's best to re-set it for 240V which is what we still actually have in the UK - to do that, swap the white/black-stripe wire that's on P4 with the black wire that's on P10, on the left-hand end of the PCB. You may need to undo the cable tie on the wiring bundle, but you can often pull the wires through as it's usually not done up very tightly.

As always, make sure you know what you're doing and that the amp is safely powered off and preferably disconnected from the wall before poking around inside.