Any opinions? I have two of the neat old Magnavox 86 chassis. One is restored with the feedback loop modified, the other has been gutted for restoration and I want to try the RH84 circuit in this one, if its worth it.
I'm gong to be creating a webpage as I go along for other DIY'ers to see how to restore/restomod these old amps as a budget Hi-fi system.
I'm also open to CCS on the input or output stages, as well as possibly negative bias ( havent heard of it really in SE amps???)
I want to keep pentode mode to keep the measly high power (triode connected doesn't give enough power at all for most normal efficiency speakers), so lets try and make a SE pentode amp sound as good as it can here.
I'm gong to be creating a webpage as I go along for other DIY'ers to see how to restore/restomod these old amps as a budget Hi-fi system.
I'm also open to CCS on the input or output stages, as well as possibly negative bias ( havent heard of it really in SE amps???)
I want to keep pentode mode to keep the measly high power (triode connected doesn't give enough power at all for most normal efficiency speakers), so lets try and make a SE pentode amp sound as good as it can here.
Wonderful high end though.
ThSpeakerDude88 said:
The one with the maggie OPTs represents available choke & OPTs installed in the last of the single chassis development (we worked thru about a dozen variations and it was probably a bit over half-way). The monoblocs have twice the powersupply, an EF86 instead of 12AT7 in the front, are parafeed (with a roll-off of about 50 Hz), and use better Grundig OPTs. They are definitely better (they took on some Audio Note Conquerers and it came out a draw (with a caveat -- the earlier roll-off complimented the room/speakers better)).
dave
Are you sugguesting the magnavox transformers as parafeed chokes? 
BTW, I am planning on wiring up this amp this week, but have experienced a major setback
while cleaning the power transformer and prepping it for paint... I dropped it and caught it by the wires-several pulled out of the end bell. Darn it! No getting these transformers apart either, their heavily potted and you CAN NOT get the screws out of the transformer!! Don't even try! I've had this happen before and they snapped off and I ended up drillng them out- which was a pain in the butt! I just tossed the tranny. Sad.
Know of any suitable replacements? The original voltages were 225-0-225 , 6.3v, 6.3v. The radio was rated at 90 watts , but I won't be drawing that much from it with just the amp.
How much current do I really need on the HT side with two SE 6bq5's and one preamp tube?
BTW, I am planning on wiring up this amp this week, but have experienced a major setback
while cleaning the power transformer and prepping it for paint... I dropped it and caught it by the wires-several pulled out of the end bell. Darn it! No getting these transformers apart either, their heavily potted and you CAN NOT get the screws out of the transformer!! Don't even try! I've had this happen before and they snapped off and I ended up drillng them out- which was a pain in the butt! I just tossed the tranny. Sad.
Know of any suitable replacements? The original voltages were 225-0-225 , 6.3v, 6.3v. The radio was rated at 90 watts , but I won't be drawing that much from it with just the amp.
How much current do I really need on the HT side with two SE 6bq5's and one preamp tube?
Thats kind of what I meant, but being so small and not having much inductance already, the magnavox OPT's probably would not make good PFC's would they? I do have four of them if it would,however, how does this affect the plate load, wouldn't it be halved since you're essentially paralelling them, or does the DC cap between the plate load and the OPT eliminate this resistance mismatch?
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.
Shoog
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.
Shoog
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?
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?
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.
Shoog
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.
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.
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
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.