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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 11:13 am
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Musepro wrote:
Webber likes you to have a probe under every power tube, which makes sense to me, yet many good amp techs and tube gurus dont find it necessary. Any comments concerning this?


Well, most amps only provide a gobal bias adjustment, (I think there may be a few with adjustment for each tube. Or is that just a balance adjustment? I forget.) so theoretically you could get away with one probe or the single Fender test point. But even the best balanced tubes may not be perfectly balanced. Also as tubes age it would be a good thing to check for unequal idle currents which could signify an amp or tube problem. You could just swap a single probe from tube to tube, but a dual (or quad probe) is so much easier.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 11:27 am
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Damn. Beat me again by 2 seconds. :lol:

shimmilou wrote:
Here is a simple mod to the tone stack, just a jumper wire, to give better control of the tone, especially the mids. Although the site is for the BJr, the same mod works well on the other HRd amps. I did this to my HRDlx, HRDvl as well as BJr. With this jumper you can completely dial out the mids for a great scooped tone, just for example of tone control. The description on the site says it all. Looking at the HRDlx schematic, jumper the top mid pot connection (the side of the mid pot connected to the bass pot), to the mid pot wiper as shown in the pictures. I had to remove the circuit board and jumper on the solder side in all but the BJr, as a cap is in the way for the HRDlx and HRDvl on the component side.

http://billmaudio.com/wp/?page_id=58.


The Hot Rod Deluxe and Blues Deluxe tone stacks are basically the same and are essentially a 59 Bassman tone stack (except for the driver and load stages). One mod I am considering for my BDRI is to replace the 0.1 mfd cap used for the bass control with a 0.022 mfd cap ala the 59 Bassman (a real 59, not the reissue). It drops the bass level slightly (just turn the control up a few notches), but more importantly rolls the bass off below 60 Hz more quickly than the stock tone stack. This helps if you have a boomy bass problem. You can model the effect with the Duncan simulator pretty well. For the HRD and BDRI you need to use the Marshall model (the Fender model is Blackface). Using a very low source impedance and a very high load impedance will isolate the tone stack and let you see better what is going one. Changing the source and load impedances mostly changes the insertion loss of the tone stack and has only a minor effect on the curve shape.

shimmilou wrote:
With amps like the HRDlx that have only two output tubes, it is easy to verify a matched set of tubes without the need for probes. With amp off, unplugged from electricity, and caps discharged, measure and note the resistance of each half of the output transformer primary (red to brown resistance, and red to blue resistance), also noting that the brown wire is for V4 Plate and blue for V5 Plate (the brown and blue wires are the Plate voltage for each tube when amp is on). Then power up amp, in play and check the DC voltage drop across each half of the transformer in the same places as the resistance was measured. Then Ohm's Law gives the Plate current on each half for each output tube.


As an engineer, I prefer to take the easy way out and just measure the idle current. Its close enough for government work. :lol: :lol: :lol:

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 11:34 am
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shimmilou wrote:
>Here is a simple mod to the tone stack...
>I use a quad probe set and can check up to four tubes at once, which is much easier than swapping one probe four times with hot tubes.
>And when dealing with many types of amps, and hundreds of tubes, bias probes are more than handy.


I've checked out a few HRDlx mod sites and considered some of them. I just didn't know, and still dont know, how much time/money I want to put into it. I use a lot of good amps bands bring in and am getting to a point where I can resume investing a bit of money into guitar gear again. After hearing the results of my tube experiments, the HRDlx moved off my sell list, but if the time comes to sell it, I fear mods might depreciate it's value (even though it's already "moded" by being a head!). It's good to have these resources. I'll have a better idea of what I want to do with it after spending more time with the 6V6s and lower gain preamps.

What quad probe set do you use?

I sometimes have to tweak gear for bands and I like to do as much maintenance on my own gear as possible. Given that I'm now in the market for more amps of my own, that's even more reason to have one. After these discussions we've been having, a probe is a solid, long term investment that will be an asset for life. I see no reason, in my position, to skimp.

Thanks!

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 12:00 pm
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As with most probes, you can use your own VOM or DMM, or some probes have a built in display for current and/or Plate voltage. If you can swing it, go ahead and get all of the probes that you can with your first order (ie, four 6L6 and four EL84), as they are cheaper than ordering extra probes later. I have the QuadStage Bias Pro Q10, with four of each probe myself, and saved some money by not waiting to get the extra probes. I did not get the Plate voltage probes, I just check it at the socket which can be more dangerous. It is really easy just turning the dial to read each tube, and very accurate. I prefer this design that has the Cathode resistor actually in the tube socket instead of in the box, for better accuracy. The RCA connectors are very solid and give a better connection than some of the other probes that I saw. About $200 and you will be covered for just about any amp as far as bias probes. And for amps like the HRDlx with 6L6, or amps with 6V6, the standard two-probe set will work well. Actually, you can get by just fine with only two of each probe type (6L6, EL84), and check two tubes at a time if you have a four-tube amp.

http://www.ebay.com/itm/QuadStage-BiasPro-Tube-Audio-Amp-Biasing-Bias-Matching-Probe-Tester-Tool-/220905948902?pt=LH_DefaultDomain_0&hash=item336f0546e6

For a little extra, this model has it's own display and uses the same probes.

http://www.ebay.com/itm/Meter-ed-QuadStage-BiasPro-MQ10-Audio-Amp-Tube-Bias-Matching-Probe-Tester-Tool-/320806383267?pt=LH_DefaultDomain_0&hash=item4ab18ceea3

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