debk,
Does your 45 amp have an output transformer? If so how much voltage can it swing and how do you plan to load it?
I have been thinking recently about building a small SE amp to drive the F4 but thinking that the transformer would need to be quite different from one that would drive speakers. Seems like you would need/want a smaller impedence ratio to get the voltage up and then use the proper resistive loading to maintain the plate load. Or also wondered if it wouldn't be better to use choke loading and then capacitively couple the output from the plate.
At the moment I'm using an OTL headphone amp to drive the F4 so I can avoid the transformer issues. It has a modified futterman output stage and sounds a little warmer than the cathode follower I was using before. I'm still thinking a nice 300b se might be another step in this direction but need to understand the OPT issues to do it right.
Does your 45 amp have an output transformer? If so how much voltage can it swing and how do you plan to load it?
I have been thinking recently about building a small SE amp to drive the F4 but thinking that the transformer would need to be quite different from one that would drive speakers. Seems like you would need/want a smaller impedence ratio to get the voltage up and then use the proper resistive loading to maintain the plate load. Or also wondered if it wouldn't be better to use choke loading and then capacitively couple the output from the plate.
At the moment I'm using an OTL headphone amp to drive the F4 so I can avoid the transformer issues. It has a modified futterman output stage and sounds a little warmer than the cathode follower I was using before. I'm still thinking a nice 300b se might be another step in this direction but need to understand the OPT issues to do it right.
Sorry been busy and didn't get a chance to respond.
There are a number of internet references about using a DHT to drive a F4.
My 45 amp has a transformer. From what I have read placing a 20 ohm resistor across the output will work fairly well with this. Once I build the second amp I will try it out in my system and let everyone know how it sounds
Debra
There are a number of internet references about using a DHT to drive a F4.
My 45 amp has a transformer. From what I have read placing a 20 ohm resistor across the output will work fairly well with this. Once I build the second amp I will try it out in my system and let everyone know how it sounds
Debra
Been experimenting a little with driving the F4 with a SET amp. Still working with only one channel in my lab.
Set up is
Source---> Bottlehead Extended Foreplay III preamp modified for 6SN7's---> Bottlehead Paramount 2a3 amp --->F4 in parallel monoblock configuration.
I tried both the 8 ohm and 16 ohm tap on the output transformer and listened to both with a variety of resistors across the SET amp output (12R, 15R, 20R, 22R, all Mills 5 watt resisitors).
Not surprising the 16 ohm tap sounded better. As for which resistance across the SET amp worked best, I think the 20R or 22R sounded best but there was not a big difference between all four. When the second channel is built and listening in stereo I may be able to refine this more.
As for my listening impressions. First it had plenty of volume, louder than I would want to listen at. The F4 adds very little to the coloration of the sound, except more power and the bass was very noticbly more present. Still warm and very detailed, much like my SET amp on steroids.
I also listened to just the preamp--->F4 combination and found this worked well also but the sound was much more alive with the SET amp in the chain.
The entire system was dead quiet no hum at all, even at loud volumes.
I look forward to trying it in stereo in my actual system (300b preamp and 45 SET amp)
I am waiting on a batch of MOSFETS to match and then will build the second channel.
I am impressed with the F4, a great amp. Like my tube amps it needs to warm up for a bit to sound it's best.
Debra
Set up is
Source---> Bottlehead Extended Foreplay III preamp modified for 6SN7's---> Bottlehead Paramount 2a3 amp --->F4 in parallel monoblock configuration.
I tried both the 8 ohm and 16 ohm tap on the output transformer and listened to both with a variety of resistors across the SET amp output (12R, 15R, 20R, 22R, all Mills 5 watt resisitors).
Not surprising the 16 ohm tap sounded better. As for which resistance across the SET amp worked best, I think the 20R or 22R sounded best but there was not a big difference between all four. When the second channel is built and listening in stereo I may be able to refine this more.
As for my listening impressions. First it had plenty of volume, louder than I would want to listen at. The F4 adds very little to the coloration of the sound, except more power and the bass was very noticbly more present. Still warm and very detailed, much like my SET amp on steroids.
I also listened to just the preamp--->F4 combination and found this worked well also but the sound was much more alive with the SET amp in the chain.
The entire system was dead quiet no hum at all, even at loud volumes.
I look forward to trying it in stereo in my actual system (300b preamp and 45 SET amp)
I am waiting on a batch of MOSFETS to match and then will build the second channel.
I am impressed with the F4, a great amp. Like my tube amps it needs to warm up for a bit to sound it's best.
Debra
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Thanks Debra,
I see, it must be the BeePre. Sounds just like what I was after
"gobs of drive, that didn't use a cathode follower, an output transformer or global negative feedback "
I'll keep it in mind.
Phil
contemplating a similar setup to what debK notes above (source >> preamp >> DHT SET amp >> F4) except that my Bottlehead paramount monoblocks are using 300B output tubes in lieu of her 2A3 or 45s.
I'm not smart enough to know whether the higher output of the 300B amp (~9watts) relative to a 2A3/ 45 amp is material here or not.
of note, I intend to build the stereo 25WPC version of the F4... at least for now
would appreciate any thoughts on the subject...thanks!
I'm not smart enough to know whether the higher output of the 300B amp (~9watts) relative to a 2A3/ 45 amp is material here or not.
of note, I intend to build the stereo 25WPC version of the F4... at least for now
would appreciate any thoughts on the subject...thanks!
I have both F4 amps built and working in my system. First I tried it with the 45 amp. While it sounded ok the 45 amp was not able to drive the F4 well enough to take advantage of it. It was slightly louder, but not much of a difference.
Next I switched out the 45 amps for my 2a3 amps and the difference was remarkable. More volume than I need and still sounds very much like a DHT amp. The F4 lends virtually no coloration to the sound. I think I will leave it in this configuration for a while
Source-->preamp--->2a3 amp--->F4
I am sure it will also work very well with a 300b amp, but I have not tried it. Mine are currently torn apart to be reworked.
Like a tube amp the F4 takes a bit of time to warm up and sound it's best.
All the listening was done using the same source: Sota Sapphire, Kiseki Purpleheart cartridge, Bob's devices SUT, Bottlehead Eros phono stage, Nora Jones LP
Debra
Next I switched out the 45 amps for my 2a3 amps and the difference was remarkable. More volume than I need and still sounds very much like a DHT amp. The F4 lends virtually no coloration to the sound. I think I will leave it in this configuration for a while
Source-->preamp--->2a3 amp--->F4
I am sure it will also work very well with a 300b amp, but I have not tried it. Mine are currently torn apart to be reworked.
Like a tube amp the F4 takes a bit of time to warm up and sound it's best.
All the listening was done using the same source: Sota Sapphire, Kiseki Purpleheart cartridge, Bob's devices SUT, Bottlehead Eros phono stage, Nora Jones LP
Debra
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Yes, it takes my F4 2hrs of warm up to get to it's full potential. For the longest time I thought it was alcohol related but then, with the help of friends we concluded it was truly thermal. After a couple of hours the entire chassis/heatsinks are at about 50 degrees C. It truly sounds it's best from that point on. A little inconvenient sometimes but worth the wait.
Thanks Debra... Appreciate the info!
I should have parts in next couple of days, so we'll see how this goes.
I presume you fabricated cables for DHT outputs >> F4 inputs. Any sage wisdom?
Bryan
I used a cable with RCA at one end and banana plugs at the other.
Put a resistor across SET amp output
I have had the thoughts cos by the time the amp is warm I am also
+1 ��