Hi,
I have the opportunity to buy a compact 6C45 SE tube amp. Sound is great but I find the overall gain too low for my needs, I feed it directly from my DAC and it plays quite low (I have no pre-amp.). I own a Darling 1626 SET and the Darling plays much louder despite it's 0.75W output. I believe the 6C45 has an output rated at 1.2W
How could I increase the gain while maintaining the overall nice SQ ?
I'm attaching the schematic if this can help, value were measured earlier, the circuit is as built. I have no clue about the OPT, they look like what was used for the bass on the RCA console (3 channels amp). Last night I added the Cathode bypass cap (220uF) and the gain went up but still not enough.
Thanks in advance,
Eric
I have the opportunity to buy a compact 6C45 SE tube amp. Sound is great but I find the overall gain too low for my needs, I feed it directly from my DAC and it plays quite low (I have no pre-amp.). I own a Darling 1626 SET and the Darling plays much louder despite it's 0.75W output. I believe the 6C45 has an output rated at 1.2W
How could I increase the gain while maintaining the overall nice SQ ?
I'm attaching the schematic if this can help, value were measured earlier, the circuit is as built. I have no clue about the OPT, they look like what was used for the bass on the RCA console (3 channels amp). Last night I added the Cathode bypass cap (220uF) and the gain went up but still not enough.
Thanks in advance,
Eric
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If the output impedence of your source is low, you may try to add a step-up transformer at the input. I had good results with the OEP z218a2e (RS components 667-6076) wired with the two primary windings in parallel and the two secondaries in series. Cheap but effective for my 6e5p spud. There are much better choices at higher prices, of course.
You'd have to add an input stage (tube or transformer) for the extra gain, like the Darling has.
Thanks for the suggestions, I have seen these 1:2 step up input transformer but wonder why they are required, the 6C45 has an amplification factor of 52. The 1626 Darling is understandable with an amplification factor of only 5.
I'll look into these input transformer unless I receive other recommendations.
Thanks
Eric
I'll look into these input transformer unless I receive other recommendations.
Thanks
Eric
Exactly, and step-up transformer at the input will be good solution. One should understand, that OPT must essentially decrease signal at tube's plate, otherwise one will not get acceptable plate load and output impedance of amp.
To illustrate with some numbers, assume that the OPT turns ratio is only 10 (n=n1/n2=10). In this case voltage amplification of the whole amp would be near Ku=5 (52/10), that is so so admittable (normally one would like to have voltage amplification 10...20). With Ku=5, around 20% of CD discs will not allow to achieve sound loud enough.
With n=n1/n2=10 and 8 Ohms speakers, one will have plate load for the tube only Ra = n*n*8 = 800 Ohms, that is terribly low even for 6s45pi with its internal impedance Ri near 1,1 kOhms. This tube will work good with at least Ra = 3...5*Ri, better if Ra is near 5 kOms. Output impedance with such n=10 also will be not good, Zout=Ri/(n*n)= 11 Ohms.
The things are much better with n=25, the Ra=25*25*8= 5 kOhms,
Zout = 1100 / 625 = 1,76 Ohms. This is acceptable, but voltage amplification of the amp is only Ku=52/25 = 2, that is not enough.
So, input step-up transformer seems to be most reasonable solution.
The tube is Mu=52, Rp about 1.1K. A happy load for this tube at 133V 0.033A might be 3K. The gain grid-plate is then almost 3/4 of 52 or 39.
Yes, the 3K:8r(?) OT is 20:1 so the gain is not quite 2 from jack to speaker.
The input signal must be near 2.5V peak (1.8V RMS) to slam "all" the output. I would think most DACs would do this, *if* the sources are normallized to full scale.
The Darling is not directly comparable because it has two stages. Two stages cascaded is LOTS more gain than one stage, even two low-gain stages. Say the 6J4 has gain of only 30 (half its Mu), the 1626 a loaded gain of 3: that is a gain of *90* jack to OT, more than double the 39 we got with the one-stage 6C45.
Sadly, you want a "little more" gain. Tubes (or transistors) tend to give ample gain; more than you want. A line-stage with gain=5 and a vol-pot set to about 0.4 is classic.
Transformer. 2K:10K gives a good trade of DAC current into grid voltage. Mouser Electronics - Electronic Components Distributor
42TL002-RC Xicon $3
148Q Hammond $20
I have used the $3 type in line-level work with satisfaction. While not rated for bass, if driven with a low Z source (such as DAC) they do well down below most loudspeakers. I do not know if the $20 job is better, but it isn't a lot of money.
Yes, the 3K:8r(?) OT is 20:1 so the gain is not quite 2 from jack to speaker.
The input signal must be near 2.5V peak (1.8V RMS) to slam "all" the output. I would think most DACs would do this, *if* the sources are normallized to full scale.
The Darling is not directly comparable because it has two stages. Two stages cascaded is LOTS more gain than one stage, even two low-gain stages. Say the 6J4 has gain of only 30 (half its Mu), the 1626 a loaded gain of 3: that is a gain of *90* jack to OT, more than double the 39 we got with the one-stage 6C45.
Sadly, you want a "little more" gain. Tubes (or transistors) tend to give ample gain; more than you want. A line-stage with gain=5 and a vol-pot set to about 0.4 is classic.
Transformer. 2K:10K gives a good trade of DAC current into grid voltage. Mouser Electronics - Electronic Components Distributor
42TL002-RC Xicon $3
148Q Hammond $20
I have used the $3 type in line-level work with satisfaction. While not rated for bass, if driven with a low Z source (such as DAC) they do well down below most loudspeakers. I do not know if the $20 job is better, but it isn't a lot of money.
I would only add, from practice of many people at Russian diy forums with just 6s45pi, it sounds bad with Rp around 3*Ri, 3-d, 5-th and 7-th harmonics are plenty at this condition.
Sound is very good at Rp not less than 5*Ri. This is because specifity of output plate curves for this tube.
So confused...help please.
Hi,
Did some progress...I borrowed my friend's pre-amp and realized it has the perfect amount of gain. His preamp has a gain (Av) of 5.3
Based on this, does this mean I'm looking for a 1:5 matching transformer ? I must say that I'm pretty confused with all this
Edcor seems to have quite a few different type, their 1/4W seems OK.
Their 1:5 model is 600 ohm to 15K ohm, would this work ? (Price is great but delay is sooo long) Please see here EDCOR - PC Series
Seems that only some microphone transformer have such a high ratio.
Any guidance would be appreciated. My DAC is the IQAudio Pi-DAC+ with MoOde Audio.
Thanks a lot,
Eric
Hi,
Did some progress...I borrowed my friend's pre-amp and realized it has the perfect amount of gain. His preamp has a gain (Av) of 5.3
Based on this, does this mean I'm looking for a 1:5 matching transformer ? I must say that I'm pretty confused with all this
Edcor seems to have quite a few different type, their 1/4W seems OK.
Their 1:5 model is 600 ohm to 15K ohm, would this work ? (Price is great but delay is sooo long) Please see here EDCOR - PC Series
Seems that only some microphone transformer have such a high ratio.
Any guidance would be appreciated. My DAC is the IQAudio Pi-DAC+ with MoOde Audio.
Thanks a lot,
Eric
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Seems to me, post #8 already gave you the most optimal solution, a Square Root of 5 (aka: 2.24:1 or 2K:10K) inter-stage transformer likely is your best bet.
If your DAC source don't have the current like your friends' line pre-amp does, you could end up with less power using a 5:1 transformer. When a transformer increase voltage it will decrease current. Nothing is free.
If your DAC source don't have the current like your friends' line pre-amp does, you could end up with less power using a 5:1 transformer. When a transformer increase voltage it will decrease current. Nothing is free.
Other easily available options are from Cinemag and Jensen. Cinemag is cheaper than Jensen, and often offers better value for the money, but has a longer lead time. Both will be more expensive than Edcor, but will also offer more "exotic" options like high nickel laminations.
Edit: Lundahl offerings are quickly available from K&K. They are usually considered to be in the same league as Cinemag and Jensen.
Edit: Lundahl offerings are quickly available from K&K. They are usually considered to be in the same league as Cinemag and Jensen.
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