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Old 10th December 2005, 02:32 PM   #1
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Default 12a?7 in Eico HF-12

I'm a newbie and have really gotten the tube bug. I purchased an old Eico HF-12 mono el84 amp to learn and play with, mainly because it was such a simple design. It came with old british Mullard tubes. The sound was quite good for such a small amp. Although the tubes all tested good, I purchased some new JJ output tubes. I got alot better base but the top end was not quite as good. The preamp tube was a 12AU7.I downloaded a schematic and in it I saw a 12AX7 preamp tube. Apparently they made the amp with the 12au7 for one or two years then switched. Being a newbie I looked no further and stuck a 12AX7 in my amp. The sound was much better and the upper end really came alive. With the old Mullard outputs in place I did get some distortion but not anything noticable with the JJs. I posted this finding on another forum and got several replies that this was dangerous and could harm the amp. I checked the voltages and with the 12ax7 the voltages were only 2-5 volts higher and still within the amp and the tubes specs (this amp really pushes the output tubes to the top of the range anyway so it was close). The 12ax7 has a gain of 100 compared to the au7s 20 but The voltages I checked ( per the manual on all tubes)hardly changed at all and I have used it for over a month now with no noticible probllems at all. Even though I asked several times on the other forum, no one ever attempted to explain what to check for or expect from this change.
Could someone please explain what is happening inside the amp with this 5 fold gain increase and what I should look for / change. other than the voltages??
I know that this is a very basic subject but I haven't been able to determine exactly what gain is or does.
Thanks

Steve
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Old 10th December 2005, 03:12 PM   #2
anatech is offline anatech  Canada
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Hi Steve,
This gain is partially a characteristic of the particular tube you are using. It's partially the circuit as well.

The tube will have a plate impedance in a range depending on the operating point (standing voltage drop and current). This interacts with what the circuit is set up for. The tube will either be operating in a linear spot of its plate characteristics, or not.

I think it was suggested to you that you should look at the circuit values to determine what the proper tube was. If the values are the same and the schematic shows a 12AX7, you have the correct tube installed.

You should be aware that old components can shift value or become leaky (caps). This may shift the actual operating point so that another tube now fits the circuit. But there is a problem that should be corrected for the circuit to operate as the designer intended.

Does that help at all?

-Chris
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Old 10th December 2005, 03:45 PM   #3
SY is offline SY  United States
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I've never seen an HF12 with anything but 12AX7. Maybe some previous owner was trying some tube-rolling?

In any case, the schematic is on the web at the Unofficial Eico Home Page, so take a look and check the values of resistors, caps, etc, and see if they correspond. If so, enjoy your 12AX7s.
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Old 10th December 2005, 07:59 PM   #4
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I have three HF-12's myself. One has 12au7 on the tube plot that was crossed out by a previous owner. I found a 12ax7 in the socket when I got it.
I haven't been able to determin what, if any, difference there is by looking into the layout. I have a schematic showing a 12ax7 in the circut.
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Old 10th December 2005, 08:06 PM   #5
anatech is offline anatech  Canada
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Hi bluelobster,
How do the serial numbers compare to the one that showed a 12AU7?

-Chris
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Old 10th December 2005, 10:18 PM   #6
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The tube diagram on the amp itself shows a 12au7 there. All the schematics I have found show the 12ax7.
My amp has the following pate resistors:
sec a= 150k,>>>>>ax7 schematic shows 33k going there<<<<<
sec. b= 100k, same as ax7 setup shown on schematic.
The coupling cap on mine (between the two sections) is .1 600v. the ax7 schematic shows .25
My setup looks the same as the pre amp on the HF-81 schematic (1 channel) only flip floped. sec a and b are reversed.

steve
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Old 6th March 2006, 10:23 PM   #7
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!!

An older version of the HF-12 had the 12AU7 like the HF-81. Some guys have some on the EICO Yahoo group

EDIT:

Oh, you found it...
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Old 24th March 2006, 03:52 PM   #8
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Smile 12AX7 question

If the circuit was designed for a 12AU7 and you put in a 12AX7, you would almost certainly get significant distortion at high volume levels, because the higher tube gain would cause clipping in such a circuit. This could be readily observed at the plate with an oscilloscope. The difference in gain would also cause some distortion all the time due to the bias not being optimal. If the distortion is not obvious, the circuit is almost certainly designed for teh 12 AX7.
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