Hi, my amplifier experience is limited to building an akido preamp. I can follow a schematic and solder decently, but I have minimal knowledge with regards to amp design and circuits.
I picked up a 300b amp second hand, built by a local (Singapore) tube amp builder. It uses a nice lundahl power transformer (350v) and has hammond 125FSE output transformers. It also has a hammond 193H, 5H choke. It uses a 5AR4 rectifier and 6sl7 as driver tubes.
I've hooked it up and it plays fine and sounds decent. But it's nothing special, it lacks finess. Also its quite loud, feels like it has too much gain. I use a pair of Fostex FE207 speakers.
I have attached a picture of the circuitry, which is beyond my ability to decipher at the moment.
Just 1 question for you guys. Should I keep it and try to modify it or sell it and move on ? Is the 6sl7 the source of all the amps problems ?
I picked up a 300b amp second hand, built by a local (Singapore) tube amp builder. It uses a nice lundahl power transformer (350v) and has hammond 125FSE output transformers. It also has a hammond 193H, 5H choke. It uses a 5AR4 rectifier and 6sl7 as driver tubes.
I've hooked it up and it plays fine and sounds decent. But it's nothing special, it lacks finess. Also its quite loud, feels like it has too much gain. I use a pair of Fostex FE207 speakers.
I have attached a picture of the circuitry, which is beyond my ability to decipher at the moment.
Just 1 question for you guys. Should I keep it and try to modify it or sell it and move on ? Is the 6sl7 the source of all the amps problems ?
Last edited:
for someone 300b amp is big name . so need good opt and good valves for drive and good parts like pot .need experience in dht valves .
If you don't like the 300b I would simply resell the amplifier, it is highly regarded by the diy community and should find plenty of buyers also considering the high quality transformers.
Full circuit schematics and better pictures are needed, but I guess that improving it would not be a easy job. If you don't like to invest money and time to get into tube amp building/modding, I suggest to sell it. The 125FSE output transformer belong to the "budget" Hammond product line and is likely the major roadblock toward a substantial sound improvement. The yellow electrolytic capacitor you pictured seems to be built 20+ years ago, so maybe a global health check of the amplifier is also needed. I've always had fun on projects like this one, but the happy end is far from granted.
you need to identify the tubes. look up their pin-outs, with it unplugged and power supply caps confirmed discharged, draw and trace the circuit (a multimeter would help) It has to be simple. (or ask the builder if available for details) That Hammond output transformer may be "adequate". If it rolls off on the top like their 1650 series and used without loop feedback , then may sound "soft" - maybe a small bypass cap on an unbypassed cathode resistor of a gain state would correct the top end. Frequency response does matter.
what tubes are in it besides the 300B? If id doesn't exhibit much hum then that's a good sign.
what tubes are in it besides the 300B? If id doesn't exhibit much hum then that's a good sign.
Last edited:
6L6 driver tube?
Looks like a tube that has a pin 7 & 8 filament.......probably a 6SL7. It also appears to have one triode as a common cathode gain stage directly coupled to a cathode follower to drive the 300b.
My observations and I could be wrong.
Looks like a tube that has a pin 7 & 8 filament.......probably a 6SL7. It also appears to have one triode as a common cathode gain stage directly coupled to a cathode follower to drive the 300b.
My observations and I could be wrong.
No worries mate. If you have enough gain you can try and remove the cathode bypass cap for the 6SL7.
Maybe you could add a negative rail for the follower, that way you can AC couple the first VAS stage to the follower and DC couple the follower to the 300b, this also changes the 300b to fixed bias. What is the operating point of the 300b?
Maybe you could add a negative rail for the follower, that way you can AC couple the first VAS stage to the follower and DC couple the follower to the 300b, this also changes the 300b to fixed bias. What is the operating point of the 300b?
From the photos this looks like a cheap, low quality build. I hope the OP did not pay too much for it. As others have commented, some decent transformers, capacitors etc, would help. But also the layout appears very poor with many connections direct to the chassis. This is not a good place to start.
If I had bought it, I would strip it and rebuild from scratch, replacing transformers after I had got the layout closer to something that should be able to give good results.
If I had bought it, I would strip it and rebuild from scratch, replacing transformers after I had got the layout closer to something that should be able to give good results.
Thanks for the honesty. I wouldn't mind doing a complete rebuild but using the current transformers with an upgrade of the iron later on if I like the way the amp sounds. Do you have a schematic I could use that would be able to use the transformers that I already have?
I'm familiar with those Hammond OPTs. They're a bit coarse in sound quality - the sound is there but not so refined. It's a shame that the OPTs aren't Lundahl - Hammond would have been fine for the power transformer and chokes. A Lundahl LL1664 would be a step up in sound quality. If you're in Asia, maybe look at some of the local Asian makers like Silk or whatever for a 3K to 5K transformer at minimum 70mA, preferably 80mA or 90mA. I would swap the 6SL7 for two 6SN7. There are LOADS of schematics for a 3 stage 300b amp with two sections of a 6SN7 as input.
Hey thanks for the feedback. Will definitely do as advised. The problem with there being loads is that I don't know where to start. Would you mind picking out one or two that would be worth pursuing?
This is just my two cents, but here goes nothing...
First of all the amp is rather poorly built. Using the chassis as a ground return path for the circuit isn't exactly the best grounding practice known to man. The chassis should be separately grounded at a single point, and separate wiring used for circuit ground.
Also, a 6SL7 is really quite a wimpy driver for a 300B; simply not enough current. As suggested a 6SN7 would be much better, or a 2-stage amp with a pentode first stage.
But as bad a reputation the Hammond OPT's have, I'd advice against just blindly ditching them. They're perfectly good for a better attempt. Their biggest limitation is their low end, but keeping that in mind, you can make a perfectly enjoyable amp with them. I've used 125DSE, 125ESE and 125FSE, and apart from some ultrasonic resonances in some examples, their top end wasn't all that bad. So I'd start experimenting a better circuit using the Hammond output iron, and then - maybe - if you manage building something already good enough, try some more expensive OPT's.
First of all the amp is rather poorly built. Using the chassis as a ground return path for the circuit isn't exactly the best grounding practice known to man. The chassis should be separately grounded at a single point, and separate wiring used for circuit ground.
Also, a 6SL7 is really quite a wimpy driver for a 300B; simply not enough current. As suggested a 6SN7 would be much better, or a 2-stage amp with a pentode first stage.
But as bad a reputation the Hammond OPT's have, I'd advice against just blindly ditching them. They're perfectly good for a better attempt. Their biggest limitation is their low end, but keeping that in mind, you can make a perfectly enjoyable amp with them. I've used 125DSE, 125ESE and 125FSE, and apart from some ultrasonic resonances in some examples, their top end wasn't all that bad. So I'd start experimenting a better circuit using the Hammond output iron, and then - maybe - if you manage building something already good enough, try some more expensive OPT's.
I think I would start by tracing out the circuit that you have, and posting here. Then folks can comment on areas that may benefit most readily from low cost changes. Then rebuild, taking those changes and layout advice into account. I would also not change the transformers initially. There's likely to be benefit in the other changes, and they'd be necessary anyway if you want to get the best out of changing the transformers later. There would be no point in changing the transformers (expensive) while cheaper improvements are within easy reach.
There us a decent opt manufacturer in Malaysia. I am very happy with their top range 3.5K opt - I know they can fit a wide range of budgets too.
J&K Audio Design
I don't have any commercial association with them.
Ian
J&K Audio Design
I don't have any commercial association with them.
Ian
Last edited:
Personally, I would consider a mosfet follower. Driving the 300b takes a low impedance source with stones.
the 6sl7 imho is whimpy, buffering it with a 6sn7 ala aikido might help...
in fact and 6sl7/6sn7 aikido to drive the 300b might even be yummier...
if you want simple, a triode wired D3a, 12GN7, 12hl7(video pentodes) might give what you want, a 6P15p will work ok too...
in fact and 6sl7/6sn7 aikido to drive the 300b might even be yummier...
if you want simple, a triode wired D3a, 12GN7, 12hl7(video pentodes) might give what you want, a 6P15p will work ok too...
Ok guys, spent the good part of the day figuring out Eagle and measuring the circuit. If there are mistakes, I apologize in advance.
I've tried to be as accurate as possible and replicate the chassis grounds as they are. Any feedback would be appreciated.
I've tried to be as accurate as possible and replicate the chassis grounds as they are. Any feedback would be appreciated.
Last edited:
Rip everything out and start over - as mentioned above, the wiring is sub-standard... To improve the circuit, you can try the Reichert 300B:
Winding 13-14 on the power transformer should also have a centre tap that is connected to ground.
- Status
- Not open for further replies.