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 Post subject: dukane 1a775 amps
PostPosted: Dec Wed 28, 2016 8:10 am 
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I have a pair of dukane 1a775 amps that I am going over to use or possibly sell, and was wondering if anyone has any info on them? They are 100W monoblock rack mounted PA amps that use four 8417's as the main drivers, I assume with two parallel sets in push-pull. These don't seem very common compared to other dukane amps that were made, so I've been having a hard time finding out much about them, other than that the 8417 tubes are pretty hard to get, especially at anything resembling a reasonable price.

I've just finally found some info on testing the 8417's in my tube tester thanks to an old thread here.

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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Tue 03, 2017 1:34 pm 
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You might try asking about the tubes in the tubes and parts section of the forums and see if there are any readily available subs for those tubes.


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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Tue 03, 2017 8:56 pm 
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Unfortunately this is a relatively unique tube that seems to have no equivalent. Low drive needed for relatively high output.

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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Tue 03, 2017 10:13 pm 
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Dang.


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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Wed 04, 2017 5:56 am 
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The 8417 is a big tube. It can run 600 volts plate and a pair will develop 100 watts. Here is the kicker though it has a very high transconductance of hold on to your seats 23000 which means it takes very little drive on the control grid to to that. 6550 will do 100 watts but needs more drive since its transconductance is 9000 so if you want to sub it you can but rebiasing and getting the additional drive will be needed to start with. Opt will probably work out but is different impedance. The 8417 had problems due to its high performance related to instability meaning parasitic oscillations and even temperature resulting in thermal runaway in some circuits. Look at the line of kt tubes but off hand I can not give a direct sub. If I recall correctly kt150 is the biggest ever made. It can dissipate 70 watts plate and a pair will punch out in the neighborhood of 300 for real watts. The kt 88 or kt 90 or kt 120 may work but i do not have the spec sheets handy. If you sub do your homework as the bias and drive are not all that matters. The heater consumption is also a concern as well as is there room without cramping the tube so it can breathe and not melt down anything near it.
The 8417 and 6550 both have a 35 watt dissipation rating and similar heater requirements but the bias and drive are what to look at there.
Good luck.


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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Thu 12, 2017 9:14 am 
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So I've got the output transformers measured and have the following info:

primary is center tapped, no ultra linear taps.
there are two secondary windings marked 35V, that have a turns ratio of 15.55:1 (give or take), compared to the primary. These are supposed to be put in series for 70V operation.
there is a center tapped secondary marked 25V that shows a 22.2:1 turns ratio.
there is a feedback winding that has a ratio of 44.4:1

Some questions regarding output transformers:

what happens to primary impedance if I put those two 35V windings in parallel instead of series?
if I use only half of the 25V winding (which would allow primary to essentially have higher impedance to the output stage), am I going to need to de-rate the power handling capability of the transformer?

is the feedback winding possibly usable as an output winding instead? that gives the same nominal impedance to the primary, though it would give an impedance of close to 16K!

I'm looking at other tubes that I could use with these transformers, since a quad of 8417's aren't so easy to find, let alone two sets of them. since I have chassis and output transformers for free, I'm trying to decide what I could do to make a useful set of amps that put out anywhere from 30-100 watts each into 8 ohms (which will more than adequately drive my speakers).

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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Thu 12, 2017 7:26 pm 
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the thing about Dukane is they are more than likely PA frequency response (overhead multiple speakers/department store quality sound), and I think you'll be disappointed in the sound quality.

Use the 25v tap for 8 ohm as a test before you go any further.

I hate to be a downer, but you would be better off starting from scratch, dumping the Dukane OT and rebuilding the amp using HI FI o/t.


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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Thu 12, 2017 7:47 pm 
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Depends. They did make some projector related stuff so if this is something related to projection equipment it might have a good frequency response.


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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Thu 12, 2017 11:20 pm 
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Only DuKane projection equipment I know of is 35mm filmstrip projectors with built-in phonograph for "synchronized" sound. Very lo-fi. No motion picture equipment. Projector amplifiers are like PA amplifiers anyway -- frequency response of a traditional film soundtrack is only 50-7000 Hz.

-David


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 Post subject: Re: dukane 1a775 amps
PostPosted: Jan Fri 13, 2017 12:11 pm 
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Artcurus wrote:
the thing about Dukane is they are more than likely PA frequency response (overhead multiple speakers/department store quality sound), and I think you'll be disappointed in the sound quality.

Use the 25v tap for 8 ohm as a test before you go any further.

I hate to be a downer, but you would be better off starting from scratch, dumping the Dukane OT and rebuilding the amp using HI FI o/t.


I had that in mind, but others who had similar bogen amps were able to get decent sound out of them. mostly this is a learning experience, and worst case I sell them to someone to use as guitar amps. My plan was to restore one to original state, and use parts from the other to try something else... compare the sound quality, and decide what to do from there. Keeping all the parts it shouldn't be that hard to put it back together as original.

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 Post subject: Re: dukane 1a775 amps
PostPosted: Aug Mon 27, 2018 6:37 am 
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Do you still have, and want to tinker with, or sell, those Dukane amps?
I have several of them and can provide a schematic.
I have designed a performance upgrade to them that changes them to 70W, class A. Not quite McIntosh, but close (output transformer is the limitation).
It includes power supply conversion to choke input, adding output tube feedback and changing output tubes to 6550 .
There's a lot of ancillary work required to make it happen.
If you want to sell them, for me, It's only worth doing if I can get them relatively cheap, and if they are in good cosmetic and mechanical condition:
$150.00@, including shipping. You can keep the 8417s.
I can provide a schematic of the redesigned amp circuit, but this is NOT a hobbyist/amateur level project. It's a lot of work and requires a fair added investment in parts.
600Volts DC, peaking at double that on the plates, is no joke. If you slip, you would be lucky to just burn holes in your fingers. I know!

Tom G


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 Post subject: Re: dukane 1a775 amps
PostPosted: Aug Tue 28, 2018 4:08 am 
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tgarson wrote:
Do you still have, and want to tinker with, or sell, those Dukane amps?
I have several of them and can provide a schematic.
I have designed a performance upgrade to them that changes them to 70W, class A. Not quite McIntosh, but close (output transformer is the limitation).
It includes power supply conversion to choke input, adding output tube feedback and changing output tubes to 6550 .
There's a lot of ancillary work required to make it happen.
If you want to sell them, for me, It's only worth doing if I can get them relatively cheap, and if they are in good cosmetic and mechanical condition:
$150.00@, including shipping. You can keep the 8417s.
I can provide a schematic of the redesigned amp circuit, but this is NOT a hobbyist/amateur level project. It's a lot of work and requires a fair added investment in parts.
600Volts DC, peaking at double that on the plates, is no joke. If you slip, you would be lucky to just burn holes in your fingers. I know!

Tom G


Tom, question,

Would 807 work in this situation?


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 Post subject: Re: dukane 1a775 amps
PostPosted: Aug Tue 28, 2018 6:04 am 
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807 is more similar to 6550 & 6L6. Gm is too low by half, compared to 8417, plus its plate is on top. 8417 plate is a socket pin.

Having said that, the modifications that I do to use 6550 would probably accommodate the 807, with minor variation, but why?
807 brings nothing to the table over 6550.

I have used EL34 in place of 8417 with good success, but still required similar mods to what is needed to make 6550 work.

TG


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 Post subject: Re: dukane 1a775 amps
PostPosted: Aug Tue 28, 2018 6:19 pm 
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tgarson wrote:
807 is more similar to 6550 & 6L6. Gm is too low by half, compared to 8417, plus its plate is on top. 8417 plate is a socket pin.

Having said that, the modifications that I do to use 6550 would probably accommodate the 807, with minor variation, but why?
807 brings nothing to the table over 6550.

I have used EL34 in place of 8417 with good success, but still required similar mods to what is needed to make 6550 work.

TG


Just curious. With 8417 unobtanium, I was just wondering 807 would work. I had a Bogen that used 807, with over 1,000 v on the plate. It was a 120 watt amp.


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 Post subject: Re: dukane 1a775 amps
PostPosted: Aug Tue 28, 2018 8:35 pm 
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My Bogen MO100A, which now runs EL34s but originally used 8417s, has a B+ of 660Vdc, certainly high enough to be deadly. This is absolute max for 8417, EL34 and 6550.

1kV definitely necessitates a top cap plate tube as arcing would happen within any base or socket not made of ceramic that included the plate, and there would still be pin to pin arcing.

As previously discussed by me and others: 807 is not a "drop in" sub for 8417, even taking into account the different plate connection.

6146B is also a possible alternative to the 8417, with the required change to the plate connection. It would also require circuit mods but might be better suited than EL34 or 6550. Never tried it. Just thinking out loud.

A new 8417 isn't exactly unobtanium, just ridiculously expensive. If anyone is interested, I have a few used 8417s that are in very good condition, measuring new spec Gm and with getters that look pristine. They are pulls from amps that I have converted to 6550 or EL34. I would sell them for $150.00 per (not matched) pair, plus shipping.

TG


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 Post subject: Re: dukane 1a775 amps
PostPosted: Aug Wed 29, 2018 4:26 pm 
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RCA used the 6146 in some of their PA and drive-in theatre amplifiers, but ran into trouble with them going into parasitic oscillation. They came out with a modification to convert the amps to use 6550s, which must have been embarrassing for them.

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 Post subject: Re: dukane 1a775 amps
PostPosted: Aug Wed 29, 2018 5:42 pm 
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6146 was the original tube used in Ampeg SVT, a 300 watt tube musical instrument amplifier.
Dukane 1A475 used 7867/6CD6GA.
Parasitic suppression plate resistors are used in both amps.
I would probably use chokes, but some sort of local RF degeneration is required.

Interesting note:
Based on examination of the plate curves, 7867/6CD6GA tube might be a more than an acceptable 8417 alternative. Changing the plate connection (including adding 10 to 50 ohm plate resistors, or small chokes), and the bias voltage, would be required, but they might work with little or no other circuit modification. The benefit is they are cheap, currently selling on eBay for around $20.00 per pair.

TG


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 Post subject: Re: dukane 1a775 amps
PostPosted: Sep Thu 06, 2018 5:42 pm 
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My friend Tom rebuilt my 1a775 around 10 years ago and he converted it to a bass amplifier which we used for the last 9 years in my country rock band when it was active...We replaced the OPT with a special NOS Schumacher transformer made for Fender back in the late '70's or 80's made to run a quartet of 6550's rather than 8417's, as I had (have) a bunch of RCA 6550's I inherited from my Dad he scored in a dumpster back in the 70's
and only had a few 8417's at the time.
My friend also ran an adjusted preamp section, I believe leaving out the 12BH7? tube and replacing with a 12aX7 in the circuit...He also added a 5 band rotary EQ section on the back panel , which he wanted to re-do later, but we have never gotten around to it, as well as a master volume, input and a ¼" direct out jack for those I pad carrying idiots at gigs who insist on plugging it in to their switching power supply violent noisy modern PA systems we've been blessed with today.
I built the rack mounted amp into the case arrangement you see here for easy :lol: transport and also added all the mechanical parts (jacks, etc) while my friend did most of the complicated stuff with the circuit, re-biasing, etc...
We use the foam pad you see below amp to keep it from "rocking" off the speaker cab while in use.
The custom made "railroad" cabinet we designed and Tom built from Baltic Birch uses (2) Eminence 15" massive cast speakers and at "normal" listening level can knock a post/pier cabin off it's foundation...In a :oops: extremely loud rehearsal things can fall off the walls!
These are great amplifiers, very nice sounding once modified and can also be used as a extremely loud PA amplifier with some pretty good "CLEAN" fidelity as well.
Ya'll have to remember DuKane/Operadio made a lot of Western Electric amplifiers for them from the end of the war through the early '70's,
so very high quality components used in their commercial and military equipment.
Most folks say it reminds them of the original 6146 Ampeg SVT (before Magnavox) and has approx. the same RMS output once modified.


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Last edited by ampstamp88 on Sep Thu 06, 2018 6:48 pm, edited 3 times in total.
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 Post subject: Re: dukane 1a775 amps
PostPosted: Sep Thu 06, 2018 6:34 pm 
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Yes, T.G., 6CD6! I've acquired quite a few over the years at rock -bottom prices, (some NOS at 50 cents each) probably the best bargain of any of the high powered used receiving tubes presently available in the marketplace...To re-emphasize the above post about "danger" and "death", this is not something even today I would take on as a rebuild myself...You have to be a bit of a technical wizard and NOT an amateur/hobbyist rebuilder like myself to modify the original circuit.


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 Post subject: Re: dukane 1a775 amps
PostPosted: Sep Thu 06, 2018 8:26 pm 
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I was sorely tempted to go with different output transformers, as the stock ones are very much lacking in the area of primary inductance. However, the way my modifications work, with a much reduced B+ and reduced output power, this is not near the liability it would be in a stock configuration.
The stock output tranny inductance is well below 10 Hy. For decent 20 Hz operation with the original 600vdc supply allowing 120 Watts, you want at least 50Hy. High power output transformers that have that much primary inductance, or higher, are just too expensive to justify.

Bass guitar (low B, not A) should actually be OK with the stock output tranny.

I use 5 bands of overlapping variable depth bridged T "notch" filters in bass preamps, along with Baxendall bass & treble controls, all of which are set to frequencies that are appropriate for a bass instrument.

At one point Bob Marleys bassist used one of my amps and enclosures for practice for an upcoming tour. He loved it, but 100 Watts was WAY too little power for the volume he played at on a big stage. I think their tech crew built him a big rack system for the tour gigs.

TG


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