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Upgrading my Dared VP-300B Tube Amplifier

yogibear

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Without feedback: The sound has sparkle in the mids / highs and feels more open and has an extended openness with energy. The midbass sounds a bit weak due to this. I think most of the sound signature is of the tube and the output transformer quality. By changing either/both the amp without any feedback will also sound quite good imo. I am planning to change the 300B tubes shortly as the current one has gone very weak. I shall post my findings post that.

Withfeedback: The sparkle and the agressiveness of the mids / highs is lost and the sound appear more balanced. This will be good for some vinyls though but not for other media like CD & youtube. I actually now am liking without the feedback and have removed the negative feedback loop. I am now so much used to the sparkle of the 300Bs and cannot live without them now. The energy of this tube is fascinating and more lively and can related to real life music which gets absent with the negative feedback loop - again imo.


Since yesterday, my right channel 300B tubes has become so weak that it shorts within 30 minutes of operation. Hence after more than 6 months i connected my Denon AV receiver to replaced the tube amplifier. I played my usual music and noticed that i could not stand not even a single track with this amplifier. The Denon sounded very very laid back, dull, non-energetic, lifeless and all negative adjectives that you can attribute to an amplifier. Needless to say i turned them off in 3 minutes and watched TV instead.

Now my worries are can i ever go back to my old way of listening music - and i think probably not. Once you get used to something more refined then looking back is not a possibility for me in hifi. Please share your experience too.
My experiences are exactly similar. SS does not seem to replicate the magic of tubes. Though I am yet to and soon will, experience what Class A with tube pre stage has to offer.

Same thinking about “no feedback” topology. You do get to hear true tubes and OPT sound signature.

And there are some rare but great tube amps with a large number of output tubes and no OPT and it’s NOT OTL design but which have a large number of output tubes with very simple, minimal component topology and you get to hear straight tubes only from input to speakers.
 
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Hari Iyer

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Same thinking about “no feedback” topology. You do get to hear true tubes and OPT sound signature.
I think the OPT is the one contributing to the distortion in the SE design. There are no cancellation of noise/ distortion in a SE and the quality of the OPT decides the final outcome of the amp. Also the tube used has a significant contribution to the final outcome in a SE design. For this reason, tube rolling makes more sense in a SE amp, In a push-pull amp tube rolling does not have that much impact.
 

nn_in

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Many have suggested me to change the stock AC power input cable for the amp. I am going by their wisdom and will be changing the puny AC input cable. I will be twisting three 2.5 sq.mm polycap cable for the line, neutral and earth and may be put them inside a sleeve. Its observed by others that midrange has body and the bass tight with this simplistic mod.
Iam interested to know how cables can make a difference.Will not any quality cable do the job.? iam not doubting any of the findings .

I think the OPT is the one contributing to the distortion in the SE design. There are no cancellation of noise/ distortion in a SE and the quality of the OPT decides the final outcome of the amp. Also the tube used has a significant contribution to the final outcome in a SE design. For this reason, tube rolling makes more sense in a SE amp, In a push-pull amp tube rolling does not have that much impact.
Is my understanding correct?
The circuit has more influence than the individual components on the final tone.The tube by itself has inherently low gain compared to solid state BJTs/mosfets.So it does not need much negative feedback.Secondly the circuit further reduces the gain that is almost independent of device characteristics.Then how do these components make such a big difference.
 

Hari Iyer

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The 300B grid stopper resistor uses a 1 kohm wire wound resistor which has some self inductance. This can combine with the miller capacitance and transconductance of the 300 B and create a series resonant circuit. The resistor inductance and the miller capacitance will tune it at around 2.5 MHz. There could be possible oscillation at this frequency in my power tube grid which could influence SQ.

The best way to avoid this is to use carbon composite resistors, but these are hard to find and quite noise prone, and drift value over time as experienced by others. The carbon composite have extremely low parasitic inductance. Second choice will be to use metal glass glazed resistor which again is not commonly available. Trying with FM, yogibear - yet to get my hands on it.( I require just a pair).

Alternatively, I am thinking of trying ferrite beads at the grid stopper resistor leads. Ferrite beads suppress RF from 100 KHz to 10 MHz and work as a lossy inductor in that frequency. This also falls within my resonant frequency range.

Will post updates next week after implantation.

Thanks for looking.
 

Hari Iyer

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I did two upgrades on my 300B amplifier yesterday evening,
1. Added two ferrite beads on the grid stopper resistor lead. Did not notice any degrade or upgrade on the SQ with this. There was anyway no RF hiss or howling in the amplifier before this too. Its more of a placebo for me.

2. Restored my partial negative feedback loop from the 300B plate to the grid stopper resistor of the 300B tube. Actually this part of the ckt was initially tested couple of months ago but i removed them due to improper value of resistor and capacitor. I yesterday added the exact same value of resistor and capacitor as used in the T-Rex 300B amplifier designed by John Broskie some decade and half ago. This is a huge change in way the amplifier sounds w.r.t. gain, distortion and bandwidth. Anybody who have listened to a SET based 300B would have experienced a bit higher 2nd harmonic distortion as high as 2% which gives the vocals a bit edgy tone which could extend in some recordings which could be sometime sweet but some high pitch sound could lead to listen fatigue. This partial negative feedback removes all the edginess and reduces the distortion to very low minuscule level not only on paper but also in listening tests.

I was amazed with the clean sound of the 300B which i had never experienced before in my setup and was getting used to it all these years as nothing much could be done as its the signature of the tube due to its Trans-conductance and gain. The partial negative feedback uses the input voltage swing to adjust the tube trans-conductance to prevent adjusting of the plate current / voltage w.r.t the swing and there by reducing the distortion.

I was intrigued to measure the distortion which as per the original manual was 2% THD. I had downloaded the RMAA 5.5 couple of years ago and the application was lying unused in my laptop which i put to use today. I connected the input/ output and measured the -dB range of 1KHz fundamental and its associated harmonic distortion. I noticed that the 2nd harmonics was -84dBu and 3rd harmonic was -96dBu. converting this to % scale means its 0.0062% for 2nd harmonic and 0.0015% for 3rd harmonic. This was the first version of my test and will like to also cross-verify by removing the feedback loop on the same amplifier. But listening test can for sure confirm the loss of the edginess in the vocals.
 

Hari Iyer

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I have noticed one more small change after adding the partial negative feedback loop. The amp now requires 10 to 15 minutes of warmup before they can sound it's full potential. Earlier this was not so and they sounded same turning on and with passage of time.
 

yogibear

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I have noticed one more small change after adding the partial negative feedback loop. The amp now requires 10 to 15 minutes of warmup before they can sound it's full potential. Earlier this was not so and they sounded same turning on and with passage of time.
My 6L6 SE has minimal NFB but sounds to another level after 1 hour of playing.
 

Hari Iyer

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After carefully listening for 2 weeks i removed the partial negative feedback loop from my 300B power amplifier. No doubt, the distortion is reduced a lot, but it also removes the juice and transients from the music which i use to enjoy a lot with my amplifier. Now i am listening as per original design, but have retained the ferrite beads in the grid stopper resistor leads of the 300Bs with good outcomes.
 

Hari Iyer

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Planning couple of mods this month on the 300Bs,

# 1 - 300B cathode bypass capacitor will be modified from Rubycon 100uF,250V Electrolytic to 55uF, 250V Polyester ( 33uF + 22uF) in parallel.

# 2 - 12Au7 input stage tube (pre-driver) bypass capacitor will be modified from Nichicon 100uF,50V electrolytic to 100uF,50V Unicon Bi-polar capacitor. Will also put one 1uF,100V PP capacitor parallel with this capacitor to reduce ESR.
 

Hari Iyer

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Planning couple of mods this month on the 300Bs,

# 1 - 300B cathode bypass capacitor will be modified from Rubycon 100uF,250V Electrolytic to 55uF, 250V Polyester ( 33uF + 22uF) in parallel.

# 2 - 12Au7 input stage tube (pre-driver) bypass capacitor will be modified from Nichicon 100uF,50V electrolytic to 100uF,50V Unicon Bi-polar capacitor. Will also put one 1uF,100V PP capacitor parallel with this capacitor to reduce ESR.
This was a well deserved mod and gave me sound stage and dynamics that i had thought of. The ESR of the capacitor do play a role in the overall sound stage. The ultrapath capacitor is now reduced from 30uF to 8uF and the 30uF is now paralleled with the cathode by-pass. So the total cathode bypass capacitance is now 93uF which is perfect for my setup.
 
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Hari Iyer

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Now because of all the mods done to this amplifier, the original chassis is not designed to hold so many external components and modifications and now there are many hanging parts inside the amplifier which could prove disastrous in case of an eventuality. So now i am drooling if i should go for an complete 300B rebuilt project. Yes this means a new pcb, new chassis and everything else new other than the components currently used. I will modify the circuit further to have a 5V voltage regulator for the 300B heater, add the ultrapath to the pch, have PIO capacitors for the psu, RF/EMI / additional earthing for noise removal etc etc etc. I am still drooling on this possibility and everything in planning stage. Maybe if this turns out especially well then i may go commercial on a case to case basis. Who knows. The 300B have a cult following which no other tube has and now i have gathered quite a bit of experience in getting to know, what makes the 300B tube sound so well. Hope this reubilt venture materalizes and is within my budget.
 

arao

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Now because of all the mods done to this amplifier, the original chassis is not designed to hold so many external components and modifications and now there are many hanging parts inside the amplifier which could prove disastrous in case of an eventuality. So now i am drooling if i should go for an complete 300B rebuilt project.
Hari, I have read this thread with great interest - very detailed and informative. And full of perseverance. BTW, all these mods remind me of the Ship of Theseus - https://en.wikipedia.org/wiki/Ship_of_Theseus !

What 300B tubes are you considering?


Tx!
 

Hari Iyer

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Hari, I have read this thread with great interest - very detailed and informative. And full of perseverance. BTW, all these mods remind me of the Ship of Theseus - https://en.wikipedia.org/wiki/Ship_of_Theseus !

What 300B tubes are you considering?


Tx!
Thanks for your interest in the rebuilt.
The Dared came originally with Shuguaug 300B tubes which I replaced last year December with Psavane 300B-N mesh plate tubes which is way better than the original tubes. I will be retaining the same tubes if I consider rebuild.
 
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