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Post subject: HRDX III pre-amp retube, and power bias
Posted: Fri Jul 01, 2011 4:57 pm
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Just to share a bit of my experience with this amp, and it's WAY over driven stock pre-amp.

I bought it about a week ago, after going through several other amps (Peavy, Egnater, Marshall). I've finally settled on the HRDX III as I really liked it's clean channel (and the price, GC on sale), and I could work with the reverb, and the drive channel.

First did the reverb resistor mod. Wasn't pleased with it, so re-soldered the resistor back in place (I removed the resistor completely, being an EE, I am not afraid of PCBs, and doing things right :wink: ).

Next I re-biased the power amp. From the Factory, it was at 57mv, which seemed really cold. Bumped that up to 69mv, and the tone is much fuller on clean.

Now, the best change. My local music shop had a set of matched JAN Phillips NOS mil-spec US 12AT7 tubes that a dude ordered 4 of, and only needed 2. I've been holding off, as I was afraid of the tone result. He only wanted 15 bucks each, so I grabbed em.

I first started out with a Sovatek 12AX7WB in V1, Stockers in V2 V3. But, it didn't really improve things very much (actually a bit worse). So, I popped out the stock V2 and V3 and put the 12AT7s in. WOW, much better. Overdrive, with just drive has a SWEET bluesy sound. Every note is very audible, and nothing is getting drowned out. Overdrive with More Drive can cover most any metal riff, and anything else I choose, and all of the notes are now AUDIBLE! HUGE improvement.

So, not being content, and having a bit of noise from the Sovatek (Occasional pop, and a faint oscillating fuzz, god these Sovatek's seem to be garbage) I took a look at the stock GT 12AX7 - Rs. The ones that were located in V2 and V3 were marked with a 1005 rating on the back. The V1 had a 1008 rating. I assumed this was the gain rating of each one, so I stuck one of the 1005s in the V1 Position. It sounds phenomenal now (at least in my opinion). I tried the 1008, and it made it start to sound a bit glassy. SO, I wound up with a stock GT12AX7 (1005) in V1 and 2 matched JAN Phillips NOS mil-spec 12AT7s in V2 and V3. I think I can live with this for quite a while (I've only just started playing, but a long time player was very impressed with the sound).

Once the new volume pot arrives (and the Fulltone OCD :D ) I think my rig will be done for the moment.


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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Fri Jul 01, 2011 6:29 pm
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Awesome 94sglife!

The 12AT7 in V2 possibly has the most affect on the drive channel, half of that tube is only used for the drive channel. Some members here with similar amps have also used the lower gain tubes with great success.

What do you think of the PCBs used in the III? They are reported to be better/thicker than those used on previous HRDlx amp versions. Any through hole solder connections, or is it still one sided circuit traces for the solder pads?

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Fri Jul 01, 2011 9:11 pm
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The PCBs look like they are from 1980. The tube PCB has me scratching my head. The PCB that holds the tubes, is mounted a bit wavy. But the main board isn't bad at all. All of the solder work was very clean.

I am going to re-route the wires. Having them zip tied is a bad idea, but it's taking me a bit to figure how ( or find a good photo from someone else) on how I want to route everything.


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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 10:55 am
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Next I re-biased the power amp. From the Factory, it was at 57mv, which seemed really cold. Bumped that up to 69mv, and the tone is much fuller on clean.
__ 94sglife

You probalbly mean 57 and 69 milliamp ? And for the two 6L6GT or for each ?

Because if it is for both tubes, this mean 34 ma each and it is "cool" ( 50 % power dissipation ) and could be alright if you like the tone.
And 69 ma for each is very very hot ; danger


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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 11:47 am
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Hi stratele52,

The HRDlx has one test point that is used to read the current of both output tubes. Since the test point is the reading across a 1 ohm resistor, it will be in mV, but can be directly translated to mA. So, in this case 69 mV (mA) at the test point, and roughly 435 volts on the Plate, would be about 13 watts idle, still on the cool side but better. I think that I ended up with mine at around 78 mV (mA), which is almost 15 watts idle.

I am still not sure if these particular 5881 output tubes are 25 or 30 watts, I have been considering them to be 25 watt, which would put me at 60% idle. If the tubes are 30 watts I am at 50%, so either way I'm good at 78 mV (mA). :)

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 12:10 pm
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94sglife wrote:
...I am going to re-route the wires. Having them zip tied is a bad idea, but it's taking me a bit to figure how ( or find a good photo from someone else) on how I want to route everything.


Here is how I did mine, if it is any help. It is very quiet on the clean channel, with more noise when switched to drive, but still just about as quiet as any tube amp can be. I tried several routing options, and this way was as good as any. Twisting the pairs helps, and make sure to keep the blue and brown output transformer primary wires away from the small ribbon cable going to the output tubes PCB. The larger ribbon cables can cause noise if they are too flat/close to the shielding on the back cover, so I put a bend in each one in the middle so they wouldn't lay flat on the back panel. :idea:

Image

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Last edited by shimmilou on Wed Jul 13, 2011 9:35 am, edited 1 time in total.

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 1:00 pm
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Hi shimmilou.

You are right about millivolts = milliamp on a 1 ohms resitor.

On my RCA receiving tubes manual I read for 6L6 ; 19 watts plate dissipation at 360 volts
6L6 GC ; 30 watts """""""""""""""" " 500
5881 ; 23 watts """""""""""""""""""" 400

These are maximum values . I use always 6L6 GC for the 500 volts rating 'cause almost any amps have more than 400 volts


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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 1:03 pm
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stratele52 wrote:
These are maximum values . I use always 6L6 GC for the 500 volts rating 'cause almost any amps have more than 400 volts


+1

Especially in any of the silverface ultra-linear amps. Lesser tubes have been known to melt down under sustained maximum-load conditions.

Arjay

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 5:12 pm
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That sounds right. I'm just not sure which the 5881WXT is, as I've treated it as a 25 watt 5881, that could handle higher Plate voltage. :?:

Fender refers to them as 6L6GC, 6L6WGC, 6L6GC/5881, and 6L6B in the schematic, tube label, tube chart, and manual respectively. Sheesh!

So, do you guys think that the WXT really means that they are truly a 30 watt tube?

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 5:16 pm
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shimmilou wrote:
So, do you guys think that the WXT really means that they are truly a 30 watt tube?


Somebody obviously does.

But personally I stopped believing in leprechauns and unicorns decades ago.

Arjay

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Sat Jul 02, 2011 5:49 pm
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:lol:

I do like the Sovtek tubes, but I am also skeptical as to whether or not the 5881WXT is really a mil-spec tube with a translated label, or if they are only similarly made, or similar specs, or simply a 5881 with a different base, or......?

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Tue Jul 05, 2011 10:09 am
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Personally, I think wattage ratings are useless. IMHO, there are 3 types of amps, Small 5-25, Med 30-60, and Large 60+. But at the same time, I have heard 10 watt custom amps that will eat a 5150's lunch. So.....


And yes, it is a fixed bias on the HRDX amps. So 69mv is correct. I could've gone a bit hotter, but the clean channel starts to break up a little soon when I went hotter than that.


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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Tue Jul 05, 2011 11:10 am
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Personally, I think wattage ratings are useless. -94slife
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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Tue Jul 05, 2011 11:54 pm
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+1 stratele52,

Actually, the mV/mA reading is useless, without the Plate voltage reading also. And you need to know the actual wattage dissipated by the tube to know if it is biased correctly (current AND voltage). Example, two 6L6 tubes with 69 mV test point reading is about 12.5 watts per tube at 420 Plate volts, and about 14.5 watts per tube at 480 Plate volts. Beware of sites/people that say a tube should be operated at X mA, without also telling at what Plate voltage. The wattage dissipated by the tube at idle is important, too much and the tubes will fail, not enough and the sound is bad. :idea:

So, if your HRDlx has about 430 Plate volts, and you read 69 mV at the test point, your tubes are idling at about 13 watts per tube. This is good for a 5881 (23/25 watt tube), and a little cool for a 6L6 (30 watt tube). :idea:

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Post subject: Re: HRDX III pre-amp retube, and power bias
Posted: Wed Jul 06, 2011 12:13 am
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shimmilou wrote:
Actually, the mV/mA reading is useless, without the Plate voltage reading also.


+1!

Actually, it's the critical measurement when performing this procedure since any bias adjustment simply cannot be accurately performed without it. One would have better luck locking a dozen chimps in a room filled with typewriters and valiantly praying that one of them replicates the preamble to the Gettysburg Address.

Arjay

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