Hi all.
I wanted to pass on some trial results from playing around with a Scott 299B to see what is possible with the stock iron. I had done the usual mods to this type of integrated amp from the early 60s that is commonly advised: cap and pot replacements, etc, but was never very impressed with it sonically. This amp were clearly built to a price point and everything is pushed hard to minimize the parts count & cost. So I pulled out all the stops and changed almost everything except the output tube and iron.
I shoehorned a Hammond 159Q 7H/150ma choke in to run the PS as choke input. This got the B+ down to 310V nicely and allowed triode operation of the 7189s. I am running the 7189s at 35ma each as the power transformer is designed to supply ~140ma and I don't think they designed in much of any excess capacity. The stock fixed bias arrangement I kept as it works OK and I didn't want to lose 10-12V of B+ across a big bias resistor. No global feedback at this point.
The driver is a Schmidt cathode coupled type using a 12AX7. This is simple, works well enough and both channels only took an additional 4ma from the B+. All coupling caps, four per channel, are some old Tektronics 0.1uF/600V oilers I have been looking for a project for. Resistors are various wattages of Allen-Bradley carbon comps: nothing fancy.
I pulled the phono section out and all the various crazy filtration they used to put in pre-amps in those days. Volume pots are 100K Bornes. I replaced the tube sockets as well. So stripped to the minimum: 4 7189s, 2 12AX7s and a 5AR4.
This thing is really sweet and much better then I was expecting! For $120 of parts, its improvement has the best cost to performance gain of any mod I have done. If you are sitting on one of these 60s integrated amps(6BQ5, 7189, 7591, etc), the combination of triode strapping and choke input PS is very worthwhile IMO.
Cons? Sure, always are. Top and bottom frequency extension have been sacrificed a bit to get the super sweet midrange. And I don't know how long the PS choke is going to last running LC at 100%. Its started buzzing a good bit after only 50 hours, so it might not be up to this task. Oh yeah... and output power is down to the 5W range. I can get it to crunch on my 94dB speakers at a moderately loud level but it is not as harsh as the clipping was when it was running in pentode. Still very enjoyable to listen to within its power capabilities.
Thats it. Happy soldering!
I wanted to pass on some trial results from playing around with a Scott 299B to see what is possible with the stock iron. I had done the usual mods to this type of integrated amp from the early 60s that is commonly advised: cap and pot replacements, etc, but was never very impressed with it sonically. This amp were clearly built to a price point and everything is pushed hard to minimize the parts count & cost. So I pulled out all the stops and changed almost everything except the output tube and iron.
I shoehorned a Hammond 159Q 7H/150ma choke in to run the PS as choke input. This got the B+ down to 310V nicely and allowed triode operation of the 7189s. I am running the 7189s at 35ma each as the power transformer is designed to supply ~140ma and I don't think they designed in much of any excess capacity. The stock fixed bias arrangement I kept as it works OK and I didn't want to lose 10-12V of B+ across a big bias resistor. No global feedback at this point.
The driver is a Schmidt cathode coupled type using a 12AX7. This is simple, works well enough and both channels only took an additional 4ma from the B+. All coupling caps, four per channel, are some old Tektronics 0.1uF/600V oilers I have been looking for a project for. Resistors are various wattages of Allen-Bradley carbon comps: nothing fancy.
I pulled the phono section out and all the various crazy filtration they used to put in pre-amps in those days. Volume pots are 100K Bornes. I replaced the tube sockets as well. So stripped to the minimum: 4 7189s, 2 12AX7s and a 5AR4.
This thing is really sweet and much better then I was expecting! For $120 of parts, its improvement has the best cost to performance gain of any mod I have done. If you are sitting on one of these 60s integrated amps(6BQ5, 7189, 7591, etc), the combination of triode strapping and choke input PS is very worthwhile IMO.
Cons? Sure, always are. Top and bottom frequency extension have been sacrificed a bit to get the super sweet midrange. And I don't know how long the PS choke is going to last running LC at 100%. Its started buzzing a good bit after only 50 hours, so it might not be up to this task. Oh yeah... and output power is down to the 5W range. I can get it to crunch on my 94dB speakers at a moderately loud level but it is not as harsh as the clipping was when it was running in pentode. Still very enjoyable to listen to within its power capabilities.
Thats it. Happy soldering!
We took ours and put in a front end like Eli Duttman’s El Cheapo, and then triode strapped 6C14π running in class A.
Souped up the power supply — added a choke a big tube rectifier and all poly caps.
Sounds VERY good. Puts out a massive 3.2w at clipping.
dave
Souped up the power supply — added a choke a big tube rectifier and all poly caps.
Sounds VERY good. Puts out a massive 3.2w at clipping.
dave
6C14pi specs
I see great minds think alike! (smile)
Duncan Amps doesn't have specs on the Russian output tube. Is it close to the 7189?
I see great minds think alike! (smile)
Duncan Amps doesn't have specs on the Russian output tube. Is it close to the 7189?
You guys are threatening your immortal souls by modifying a classic from The Golden Age! Just kidding, of course. Keeping these tightly strung race horses alive after half a century plus is quite a feat.
Down south in the states mains voltages continue to inch upwards, currently about 10% above design center when these OGs were built. Yet another challenge. Hope all's well in BC; we're living in interesting times. But don't worry folks, the weather's not changing!
All good fortune,
Chris
Down south in the states mains voltages continue to inch upwards, currently about 10% above design center when these OGs were built. Yet another challenge. Hope all's well in BC; we're living in interesting times. But don't worry folks, the weather's not changing!
All good fortune,
Chris