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Old 18th April 2008, 04:21 PM   #11
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Correct choice of the parafeed cap can allow you to boost the bass where it naturally starts to roll off. Experimental changing of the cap is needed to find the right boost point.
However my experience (with a RH807 parafeed amp using microwave oven transformers as grid chokes) having such low inductance in the bass region may introduce phase shift issues. which can sound a little odd.

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Old 20th April 2008, 03:33 AM   #12
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I think I'm just going to try the RH84 and see what happens, its a simple enough circuit. Any sugguestions on mods or is it nice the way it is?

As a matter of thought: I have not played with my feedback loop much in the magnavox, other than removing the .1uf/33k part from the loop and using the 68k resistor only. I wonder if I am having some unseen oscilation which is causing my power transformer to heat up real bad? Or does this usually only happen in PP amps?
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Old 20th April 2008, 04:10 AM   #13
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Quote:
Originally posted by ThSpeakerDude88
I think I'm just going to try the RH84 and see what happens, its a simple enough circuit. Any sugguestions on mods or is it nice the way it is?
It so far is the best small SE EL84 we have tried. The next thing we want to try is regulated screens.

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Old 20th April 2008, 09:29 AM   #14
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It so gar is the best small SE EL84 we have tried. The next thing we want to try is regulated screens.
Its worth a try, but my experience with the RH807 suggests you will hear no difference. I tried triode strapped, +B referenced and cap coupled screens, and could here very little difference in any of them. I think the triode strapped version was fractionally smoother. The Partial feedback works in remarkable ways.

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Old 25th August 2008, 02:42 AM   #15
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I just finished up the conversion ( haven't really had time recently with school starting back up.)


Initial testing- WOW

This is the FIRST amplifier I have used that I do NOT Have to use any sort of EQ'ing to get the responce right, its pretty flat and the bass responce is extraordinary for such a small amp! I thought the OPT's might limit it, but alas I tried a 8k 15w edcor and it did not change the LF extension. I have a tad of hum, I don't know if it is attributed to the lack of a choke or the fact that I only have 200 ohm cathode resistors on the outputs.

For the power supply, the only deviation I made was that since I don't have a choke right now I used a 100 ohm resistor. My PT seems to be getting pretty hot as well, not sure if that is because of the extra current being drawn by the 12AT7 (~20ma as opposed to the 8-10 for the 6EU7) or because I have 100uf of capacitance after the 100 ohm resistor in the power supply

Hopefully I am not hurting the PT or rectifier by doing this, I will be ordering a choke soon.

Back to the sound: Soundstage- huge! The last time I was able to get good imaging out of my Nova 6's was when I had them on the floor about 3 feet apart with the old maggie circuit. Sounded pretty impressive, but when I put them on my desk in a corner the imaging was lost and the sound was once again "coming out of boxes." Not anymore! While I must admit Radio Shack speakers aren't exactly hi-fi, they are a really decent sounding 2 way acoustic suspension- and obviously efficeint enough to crank a little before clipping the amp. They give me tight low bass down at low volumes, and their stock tweeters don't sound bad at all.

The kind of music I listen to is Muse, Evanessence, Vanessa Carlton etc, as well as a lot of euro/dance. This kind of music has a lot of stereo mixing and reverb added, which makes for an amazing spine-shivvering experience with the right amp. I can close my eyes and feel the sound surrounding me now.


On the stock circuit, it calls for 47uf cathode bypass caps, but all I had in the parts box were 330uf. When I build my next one, should I just stick with 47uf? Also what would you reccomend as a replacement for the 12AT7? I don't have many on hand. The best to me was an RCA grey plate and a new JJ ECC81. I tried a Fender labled sovtek LPS and it sounded pretty muddy.

What other tubes have enough current drive and mu but better linearity than the 'At7?

As a last remark, I think that everyone should have one of these little maggies in their house. I want one for every room! Unfortunatly, word on these amps is starting to get out and prices are up over $70-$100 for one unmodified.
I guess it still is ok though, as the OPT's are good, the PT decent, and you usually get either a Mullard 6EU7 out of the deal , or if you don't get Matsushita 6BQ5's you get the older Mag' branded Mullards.
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Old 25th August 2008, 07:25 AM   #16
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Quote:
what would you reccomend as a replacement for the 12AT7
IMHO the T7 has been very carefully selected for this application and works very well.

After quite a bit of tube rolling (which included 5965s) the Sylvania Gold brand 6201 emerged as my clear favourite. JJs are not even close in this circuit.
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Old 26th August 2008, 02:42 AM   #17
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Well my power transformer is definatly getting hotter than it used to. Is it the 12AT7 causing this, or the 220uf capacitance without a choke?
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Old 26th August 2008, 03:20 AM   #18
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Could you please clarify, are you modding a P-P amp, I'm not quite sure which amp is an "86" chassis. Is that the P-P 6BQ5? I have experience with the little Maggie SE EL84 driven by 6EU7, and I have two of the P-P 6V6 amps. The SE is my daily listener in the garage. I reduced some of the feedback components, and actually lowered the bypass cap on the EL84's to Solen 5.1uF, after listening to various other electrolytics. I felt the transient response was better with the smaller caps, I think that's the right term, HF response better. I use some Toshiba 2way speakers that were basically open baffle thrift store finds for low power solid state. I made one channel of an RH84 once, but I wasn't sure I really liked it that much, wasn't more audible power. However I didn't get to trying the CCS on the outputs. Even though Maggie opts are small, I'm not so much a bass addict as much as an overall listening experience. my 2cents
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Old 26th August 2008, 03:24 AM   #19
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BTW almost forgot, what is your B+ on the 6BQ5's now? It may be quite a bit over 250 with the low impedance in your power supply. Probably not a problem for the outputs, but may be passing more current than your power transformer was designed for.
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Old 26th August 2008, 04:03 AM   #20
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278 off of the first filter node, and 265 on the plates. Screens are around 268 or so, and the voltage on the at7's plates is 145. Bias on the 6bq5's is a steady 7v for each, measured cathode to ground.
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